Science

(b) Dependent Variable

The dependent variable is the one that is measured as the result of the experiment; the experimenter does not control it directly, because it changes in response to the independent variable.

(b) momentum

Momentum $= mass \times velocity$, so its unit is $\text{kg}\times \text{m/s} = \text{kg m/s}$.

(d) 250 N

$F \propto \dfrac{1}{d^2}$. Doubling the distance makes the force $\dfrac14$ as large:

$F = \frac{1000}{2^2} = 250 \text{ N}$

(a) 3N

By Archimedes principle, the weight of water displaced equals the loss of weight in water: $10 - 7 = 3$ N.

(d) 2

In $XCl_2$ one atom of X combines with two Cl atoms (each of valency 1), so the valency of X is 2.

(b) Glycerol

Glycerol (propane-1,2,3-triol) has three $-OH$ groups. Methanol and ethanol are monohydric, and ethylene glycol is dihydric.

(b) Phylum, Class, Order, Family, Genus

The hierarchy runs Kingdom, Phylum, Class, Order, Family, Genus, Species. Option (b) keeps this order, while the others skip or reverse levels.

(b) $100^\circ$C, $212^\circ$F and 373 K

The upper fixed point is the boiling point of water at standard pressure: $100^\circ$C $= 212^\circ$F $= 373$ K.

(c) $A_2B$

A ($1s^2 2s^2 2p^6 3s^1$) is sodium; it has 1 valence electron and loses it, so valency 1. B ($1s^2 2s^2 2p^4$) is oxygen; it has 6 valence electrons and gains 2, so valency 2. Cross-multiplying the valencies gives $A_2B$ ($Na_2O$).

(a) The gametophytic stage of Bryophyta is dominant, while the sporophytic stage of Pteridophyta is dominant.

In mosses and liverworts the leafy gametophyte is the main plant, whereas in ferns the visible plant is the sporophyte.

(c) Decrease in number of red blood cell

Anemia is caused by too few red blood cells or too little haemoglobin, so the blood carries less oxygen.

(d) Sterile, smallest, feeds honey

Worker bees are sterile females and the smallest bees in the colony. They feed on honey and pollen, while royal jelly is for the queen and young larvae.

(b) Statement 1 is true and Statement 2 is false

Drones develop from unfertilized eggs by parthenogenesis and they mate with (fertilize) the queen, so Statement 1 is true. Fertilized eggs give queens (fertile) and workers (sterile), so not all are fertile and Statement 2 is false.

(d) Phenotypic 1:1 and Genotypic 1:1

Hybrid black is $Bb$ and white is $bb$. The cross $Bb \times bb$ gives $Bb : bb = 1:1$ in genotype, and black : white $= 1:1$ in phenotype.

(d) Right ventricle, pulmonary artery, lungs, pulmonary vein, left auricle

In pulmonary circulation, deoxygenated blood goes from the right ventricle through the pulmonary artery to the lungs, and oxygenated blood returns by the pulmonary vein to the left auricle.

(c) Upthrust acts on the object inside water.

Water pushes upward on a submerged body with a force equal to the weight of water displaced. This upthrust cancels part of the body's weight, so Ranjana feels lighter. Gravity and her mass do not change.

(c) A lot of water has been mixed in that milk.

A lactometer measures the density of milk. Pure milk is denser than water, so the lactometer floats higher in it. Sinking more means the milk is less dense, which happens when water is added.

(c) Blue, Green, Red

In the visible spectrum, wavelength increases from violet/blue (about 450 nm) to green (about 530 nm) to red (about 700 nm).

(a) 1:2

For a transformer,

$\frac{V_p}{V_s}=\frac{N_p}{N_s}=\frac{N_p}{2N_p}=\frac{1}{2}$

so the primary and secondary voltages are in the ratio $1:2$ (a step-up transformer).

(a) Be < Mg < Ca

Reactivity of metals increases down a group, as atoms lose their outer electrons more easily. Be, Mg and Ca are in group 2 in that order. In (b) the correct order would be Li < Na < K, and (c) and (d) include non-metals or go across a period.

(a) Methane

A saturated hydrocarbon has only single bonds between carbon atoms. Methane ($CH_4$) is an alkane. Ethene has a double bond, while ethyne and propyne have triple bonds.

(a) Both Statement A and Reason R are correct and Reason R is the correct explanation of Statement A.

Greenhouse gases trap heat and cause global warming. The rising temperature melts Himalayan snow and glaciers, so R explains A.

(a) When average density and critical density are equal

If the average density of the universe equals the critical density, the geometry of space is flat. Greater density gives a closed universe and lesser density gives an open universe.

(c) ADC converts analog signal to digital signal.

An Analog-to-Digital Converter (ADC) changes a continuously varying analog signal into discrete digital values. A DAC does the reverse. Analog signals vary continuously, not segment by segment.

(a) Ammonia gas is collected in a gas jar by displacing air downwards.

Ammonia is lighter than air and very soluble in water. So it cannot be collected over water and is collected by downward displacement of air, with the jar held upside down.

(c) Dieldrin, DDT, methoxychloride

These are all organochlorine insecticides. Potassium chloride and sodium chloride are ordinary salts, not insecticides.

(a) Copper has a higher specific heat capacity than aluminum.

Since $Q=mc\Delta T$ and $Q$ and $m$ are equal, $c\propto \dfrac{1}{\Delta T}$. Copper rises by only $15^\circ$C against $25^\circ$C for aluminum. Hence $c_{Cu}>c_{Al}$ under this experiment's data.

(c) The angle of incidence is greater than the critical angle, and light is traveling from a denser to a rarer medium.

Total internal reflection needs light to go from a denser medium (water, $n=1.33$) to a rarer one (air) at an angle of incidence above the critical angle, $\sin C = 1/1.33$, so $C\approx 48.8^\circ$.

(b) The presence of cosmic microwave background radiation

The CMB is the faint, uniform afterglow of the hot early universe. It was predicted by the Big Bang model and discovered in 1965, which makes it the strongest evidence.

(b) $NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$

In a double displacement reaction, two compounds exchange ions. Here Na and Ag swap partners, and $AgCl$ is formed as a white precipitate. (a) is combination, (c) is single displacement, and (d) is decomposition.

(c) Liquid exert more pressure on lower surface than upper surface on object.

Pressure increases with depth, so the upward force on the lower surface exceeds the downward force on the upper surface. This net upward force is the upthrust.

(d) 5N

By Pascal's law, $\frac{F_1}{A_1}=\frac{F_2}{A_2}$, so $F_1=15\times\frac{A_1}{3A_1}=5$ N.

(a) Kelvin

The SI unit of temperature is the kelvin (K). Celsius and Fahrenheit are other scales, and joule is the unit of energy.

(a) Water

A substance with the highest specific heat capacity loses heat most slowly for a given mass and temperature fall, so it stays hot longest. Water has the highest value, 4200 J/kg$^\circ$C.

(c) X - rarer medium, Y - denser medium

The ray bends towards the normal ($30^\circ<42^\circ$), which happens when light enters a denser medium from a rarer one.

The correct figure is the one in which the ray bends towards the normal on entering the glass, bends away from the normal on leaving, and the emergent ray is parallel to the incident ray, so $e=i$. The ray is also laterally displaced from its original line. Figures showing an emergent ray that is not parallel, or bending the wrong way at either surface, are incorrect.

(d) Dynamo

A dynamo (AC generator) produces alternating current, whose polarity reverses periodically. Simple, dry and solar cells give direct current with fixed polarity.

(c) Law of mutual induction.

A changing current in the primary coil changes the magnetic flux linked with the secondary coil, inducing an emf in it. A step-up transformer has more turns in the secondary than in the primary.

(b) Gravitational Force

Gravitational attraction between heavenly bodies keeps planets orbiting stars, moons orbiting planets and stars bound in galaxies.

(a) It is based on increasing atomic weight

Mendeleev arranged the 63 elements known then in order of increasing atomic weight. Arrangement by atomic number is the basis of the modern periodic table.

(c) Decomposition reaction

$2KClO_3\xrightarrow{\Delta}2KCl+3O_2$

One compound breaks down into simpler substances.

(b) $NH_3$

Ammonia gas is used to develop blue prints (copies) of maps and drawings.

(b) Silver

The ore $Ag(Cu)_2S$ contains silver as the metal named first in the formula, so it is an ore of silver (with copper).

(c) ethyne

Ethyne ($C_2H_2$) has a carbon-carbon triple bond, so it is an alkyne. Ethane is an alkane, ethene an alkene and ethyl alcohol an alcohol.

(d) Saponin

Saponin is a natural foaming agent that gives reetha its soap-like cleaning action.

(b) Amount of sunlight is independent variable and rate of photosynthesis is dependent variable.

The researcher changes the sunlight (independent variable) and measures how the rate of photosynthesis responds (dependent variable).

(b) Drag and add all video to project library.

After selecting the videos with the Add button, they must first be placed in the project library; only then can they be dragged to the storyboard, edited and finished.

(a) Organism A is hermaphrodite whereas organism B is unisexual.

A (segmented, long cylindrical body) is an annelid such as the earthworm, which is hermaphrodite. B (unsegmented, cylindrical) is a roundworm (nematode), which has separate sexes. Both phyla are triploblastic, so (c) and (d) are wrong.

(c) Gymnosperm

Cones instead of true flowers, separate unisexual male and female cones and wind pollination are characteristics of gymnosperms such as pine.

(a) Hydra, Jelly Fish, Coral

All three are coelenterates (cnidarians). Cuttle fish is a mollusc, cray fish an arthropod and star fish an echinoderm.

(a) Drone

Drones develop from unfertilised eggs, so they are haploid. Queen and worker bees develop from fertilised eggs and are diploid.

(b) Collecting the pollen of the flower.

The pollen basket is a structure on the hind legs of a worker bee used to carry pollen.

(d) Both B and C

Human male sex chromosomes are XY, so after meiosis each sperm has either 22 + X or 22 + Y chromosomes.

(d) Red Panda, Lynx, One-horned rhinoceros

All three are protected animals of Nepal. Jackal, rabbit and monkey are not protected species.

(a) Mitosis

Mitosis produces new identical body cells for growth and repair, including healing of wounds.

(b) Coronary artery

The coronary arteries branch from the aorta and supply oxygenated blood to the heart muscle.

(b) ECG

Sudden chest pain with collapse suggests a heart problem (such as a heart attack). An electrocardiogram records the heart electrical activity and detects it.

(c) Carbon Storage

Storing (sequestering) carbon, for example in forests and soil, removes CO$_2$ from the atmosphere and reduces climate change. Emission and exploitation increase it.

(b) 4.002 m/s$^2$

$g=\frac{GM}{r^2}=\frac{6.67\times10^{-11}\times6\times10^{24}}{(10^{7})^2}=\frac{4.002\times10^{14}}{10^{14}}=4.002\ \text{m/s}^2$

Here $r=10^4$ km $=10^7$ m.