Physics
A decibel is

A musical note
A measure of sound level
A musical instrument
The wavelength of noise

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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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