Which of the following requires a critical mass?
a) fission
b) gamma emission
c) beta emission
d) lepton

A) fission

Explanation:

Fission reactions require a critical mass. The critical mass is necessary because it makes a fission reaction self sustaining. This critical mass is the minimum amount of material required. Nuclear fission additionally depends on the composition, purity and concentration of the fissile material. Critical mass is important in the design of nuclear warheads.

specific gravity is the density of a substance divided by the density of water. since (at standard temperature and pressure) water has a density of 1 gram/cm3, and since all of the units cancel, specific gravity is usually very close to the same value as density

so "c" is the answer

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