Mass number = protons + neutrons
If you have the # of protons and the mass #, subtract the number of protons from the mass number to get the number of neutrons.
If you have the number of neutrons and the mass number, subtract the number of neutrons from the mass number to get the number of protons.
The answer is A) they provide a source of energy.
Umm well from my studies shown that its extintion
