Which of These Statements Concerning Atomic Mass Is Correct

Which of these statements concerning atomic mass is correct?

A) Atomic mass is the same as the mass number of the most common isotope.
B) Atomic mass is derived from the weight of the most common isotope of the element.
C) Atomic mass is determined by adding the masses of the protons, electrons, and neutrons in the atom.
D) Atomic mass is calculated from the atomic mass of the two most common isotopes.
E) Atomic mass is a weighted average of the masses of the naturally occurring isotopes.