Physics
Planets do not twinkle because

They are nearer to earth and hence we receive a greater amount of light.
Their distance from the earth does not change with time
They are very far away from the earth resulting in decrease in intensity of light
They emit light of a constant intensity

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Physics
Stationary wave is formed by

A transverse wave superposing a longitudinal wave
Two waves of the same speed superposing
Two waves of same frequency travelling in the same direction
Two waves of same frequency travelling in the opposite direction

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to expansion in summer can be avoided
Steel can be saved
Air-gaps are necessary for bearing the weight of running train
Accidents due to contraction in winter can be avoided

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