How Can a Nucleus Be Described by Particular Values of A

Multiple Choice

How can a nucleus be described by particular values of A, Z and N when the mass of the nucleus is not equal to Zm_p + Nm_n, where m_p and m_n are the masses of free protons and neutrons?

A) A, Z and N have no intrinsic meaning.
B) A, Z and N describe the number of particles of given types, but mass has no meaning when part of the mass is elsewhere in the universe.
C) A, Z and N describe the number of particles an ideal rather than a real nucleus would have.
D) A, Z and N describe the number of particles of given types in the nucleus, but not their masses in a bound state.
E) A, Z and N describe the number of particles of given types in the nucleus since the missing mass consists of electrons that are also present in the nucleus.