$m_{p}$ and $m_{n}$ are the masses of a proton and a neutron, respectively. For an element of mass $M$ having $Z$ protons and $N$ neutrons, which of the following is true?

• A
  $M > Z m_{p} + N m_{n}$
• B
  $M = Z m_{p} + N m_{n}$
• C
  $M < Z m_{p} + N m_{n}$
• D
  $M$ may be greater than, less than, or equal to $Z m_{p} + N m_{n}$, depending on the nature of the element.