Physics
A dynamo is a device which

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

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Physics
Specific gravity is defined as the ratio of

Density of water at 4°C to the density of the substance
Density of the substance to the density of water at 4°C
Density of the substance to the density of water
Density of the substance to the density of water at 0°C

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