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
The Hooke's law is valid for

Entire stress strain curve
Only proportional region of the stress strain curve
Elastic as well as plastic region of the stress strain curve
Entire elastic region of the stress strain curve

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