The Number of Neutrons Present in the Nucleus of an Average

The number of neutrons present in the nucleus of an average atom of any given element is best estimated by

A) adding the number of electrons and protons together.
B) subtracting the number of electrons from the number of protons.
C) adding the mass number to the number of electrons.
D) subtracting the number of protons from the mass number.
E) adding the atomic number and atomic mass together.