Physics
A mirage occurs because of

Total internal reflection by layers of air
Reflection by hot ground
Interference of light
Diffraction of light

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Physics
It is easier for a man to swim in sea water, than in river water because

The density of sea water is lesser than the density of river water
The sea is wider than the river
The density of sea water is more than the density of river water
The sea is-deeper than the river

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