Chlorine, Which Exists in Nature in Two Isotopic Forms, Has

Chlorine, which exists in nature in two isotopic forms, has an atomic mass of 35.5 amu. This means that:

A) All chlorine atoms have masses of 35.5 amu.
B) More than half of all chlorine atoms have masses of 35.5.
C) 35.5 amu is the upper limit for the mass of a chlorine atom.
D) The mass of one isotope must be lighter than 35.5 amu while the mass of the other isotope is heavier than 35.5 amu.