Solid lead has more than four times the mass density of solid aluminum. What is the main reason that lead is so much more dense?

(a) The Pb atom is smaller than the Al atom.
(b) The Pb nucleus is smaller than the Al nucleus.
(c) The Pb nucleus is more massive than the Al nucleus.
(d) The Pb nucleus is more dense than the Al nucleus.
(e) The Pb atom has many more electrons than the Al atom.