Physics
A dynamo is a device which

Creates electrical energy
Converts electrical energy into mechanical energy
Creates mechanical energy
Converts mechanical energy into electrical energy

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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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