The correct answer is natural selection. C is the answer!
Bacteria has a stronger resistance that usually produces even more resistant bacteria, and the cycle just repeats itself, making more and more resistant bacteria. This process is called Natural Selection.
All large radioactive atoms decay into smaller atoms by emitting alpha particles which consists of two protons and two neutrons. An alpha particle can also be thought of as the nucleus of a helium atom and as such they are sometimes referred to as He2+ . The positive charge is due to the lack of two electrons.