Physics
A hydrogen balloon floats up because of

Air pressure decreases with decrease in height
Air pressure decreases with decrease in weight
The pressure inside the balloon is more than the pressure outside it
Weight of the balloon is less than the weight of air displaced by it.

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

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