Answer: A. A proton and a neutron each have a relative mass of about 1, while an electron has a relative mass of only about \(1/1840\). The protons and neutrons sit in the nucleus, so almost all of an atom's mass is in the nucleus.
B is wrong: the nucleus and the electrons attract each other (opposite charges), they do not repel. C is wrong: protons are positive and neutrons are neutral, so they do not have opposite charges. D is wrong: the shell nearest the nucleus holds a maximum of 2 electrons, fewer than the outer shells of most atoms. Answer A.
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