Physics
A star appears twinkling in the sky because of

Diffraction of light by atmosphere
Refraction of light by atmosphere
Scattering of light by atmosphere
Reflection of light by atmosphere

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Physics
Circuit breaker usually operate under

Sub-transient state of short-circuit current
After D. C. component has ceased
Transient state of short-circuit current
Steady state of short-circuit current

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

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