Physics
The characteristic invalid for heat radiation is that it travels :

Heating the medium through which it passes
In a straight line
With the speed of light
In all directions

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

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