20. The major portion of an atom's mass consists of (A)neutrons and protons (B)electrons and protons (C)electrons and neutrons (D)neutrons and positrons

## Step 1<br />The question is asking about the major portion of an atom's mass. An atom is composed of three types of subatomic particles: protons, neutrons, and electrons.<br /><br />## Step 2<br />Protons and neutrons are located in the nucleus of an atom, while electrons orbit the nucleus.<br /><br />## Step 3<br />The mass of a proton or a neutron is significantly greater than that of an electron. Therefore, the majority of an atom's mass comes from the protons and neutrons in the nucleus.