the same as the atomic number. the same as the group number. the sum of protons and neutrons. the difference between protons and electrons.
I gave my students this multiple choice question: The atomic mass of an atom is

Only one answer is correct among these 4 choices:
- the same as the atomic number.
- the same as the group number.
- the sum of protons and neutrons.
- the difference between protons and electrons.

Could you tell me which one is correct?

the sum of protons and neutrons.