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

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

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Physics
The blue colour of water in the sea is due to

Reflection of blue light by impurities in sea water
Reflection of blue sky by sea water
Scattering of blue light by water molecules
Absorption of other colours except blue by water molecules

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