Physics
On a clean glass plate a drop of water spreads to form a thin layer whereas a drop of mercury remains almost spherical because:

mercury is a metal.
cohesion of water is greater than its adhesion with glass.
cohesion of mercury is greater than its adhesion with glass.
density of mercury is greater than that of water.

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Physics
Stars twinkle because

None of these
the intensity of light emitted by them changes with time.
the distance of the stars from the earth changes with time.
the refractive index of the different layers of the earth's atmosphere ch

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Physics
It takes much longer to cook food in the hills than in the plains, because

in the hills the atmospheric density is low and therefore a lot of heat is lost to the atmosphere
in the hills the humidity is high and therefore a lot of heat is absorbed by the atmosphere leaving very little heat for cooking
due to low atmospheric pressure on the hills, the water boils at a temperature higher than 100oC and therefore water takes longer to boil
n the hills the atmospheric pressure is lower than that in the plains and therefore water boils at a temperature lower than 100oC causing an increase in cooking time

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