Physics
Stars twinkle because

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

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Physics
It is easier for a man to swim in sea water, than in river water because

The sea is-deeper than the river
The density of sea water is lesser than the density of river water
The sea is wider than the river
The density of sea water is more than the density of river water

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