Define the mass number of an atom

The mass number of an atom, which is also called the atomic mass number,  is the total number of protons and neutrons in an atom,

Explanation:

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a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.

the awnser is group 12