Q: What makes up the mass number of an atom?


Read this paragraph and choose the correct option from the provided answers:

The mass number is the number of protons plus the number of neutrons in an atom. For example, most atoms of helium have 2 neutrons, so their mass number is 2 + 2 = 4. This mass number means that an atom of helium has a mass of about 4 amu.

 Choices:

- electrons and neutrons

- atoms and neutrons

- molecules and electrons

- protons and neutrons


A:
protons and neutrons