Question

why is atomic mass measured in atomic mass units (amu)?
electrons and neutrons make up nearly all the mass of an atom, and each is worth 1 amu. each atom of an element has a mass measured in amu; thus, their weighted average, or atomic mass, is measured in amu.
electrons make up nearly all the mass of an atom, and each is worth 1 amu. thus, their weighted average, or atomic mass, is measured in amu.
protons and neutrons make up nearly all the mass of an atom, and each is worth 1 amu. each atom of an element has a mass measured in amu; thus, its weighted average, or atomic mass, is measured in amu.
protons and electrons make up nearly all the mass of an atom, and each is worth 1 amu. each atom of an element has a mass measured in amu; thus, their weighted average, or atomic mass, is measured in amu.

Explanation:

Protons and neutrons contribute most of an atom's mass, with each approximately worth 1 amu. Atomic mass is the weighted - average of the masses of an element's atoms, so it is measured in amu.

Answer:

Protons and neutrons make up nearly all the mass of an atom, and each is worth 1 amu. Each atom of an element has a mass measured in amu; thus, its weighted average, or atomic mass, is measured in amu.