Physics
The point where total mass of a body is supposed to be concentrated is known as

Centre of motion
Centre of mass
Centre of gravity
Dead centre

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Physics
Specific gravity is defined as the ratio of

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

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