Carbon-12 Is the Most Common Isotope of Carbon and Has

Carbon-12 is the most common isotope of carbon and has a mass number of 12. However, the atomic mass of carbon is slightly more than 12 daltons. Why?

A) The atomic mass does not include the mass of electrons.
B) Some carbon atoms in nature have an extra proton.
C) Some carbon atoms in nature have more neutrons.
D) Some carbon atoms in nature have a different valence electron distribution.
E) Some carbon atoms in nature have undergone radioactive decay.