Physics
Which sources produce alternating current (AC)?

Batteries
Hydro-electric generators
All of these
Solar power plants

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Physics
Specific gravity is defined as the ratio of

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

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