Physics
Stars twinkle because

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

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Physics
Isotopes of an element contain

the same number of neutrons but different number of protons
equal number of protons and electrons
equal number of nucleons
the same number of protons but different number of neutrons

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