Physics
The optical phenomenon that is primarily responsible for the observation of rainbow on a rainy day is

Reflection
Diffraction
Dispersion
Interference

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

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