The atomic number (Z) is the number of protons. The mass number (A) is the number of nucleons. Hence, A is the sum of Z and the number of neutrons (N): A = N + Z and N = A - Z. So both atoms have 40 nucleons, atom A has 19 protons and 21 neutrons, atom B has 20 protons and 20 neutrons, and only statement B) is accurate.