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
Railway tracks are banked on curves

necessary centrifugal force may be obtained from the horizontal component weight of the train
to avoid frictional force between the tracks and wheels
necessary centripetal force may be obtained from the horizontal component of the weight of the train
the train may not fly off in the opposite direction

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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:

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

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Physics
Dynamo is a device for converting

Magnetic energy into electrical energy
Chemical energy into electrical energy
Heat energy into electrical energy
Mechanical energy into electrical energy

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