Amorphous silica, SiO₂, has a density of about 2.2 g/cm³, whereas the density of crystalline quartz, another form of SiO₂, is 2.65 g/cm³. Which of the following statements is the best explanation for the difference in density?
(a) Amorphous silica is a network-covalent solid, but quartz is metallic.
(b) Amorphous silica crystallizes in a primitive cubic lattice.
(c) Quartz is harder than amorphous silica.
(d) Quartz must have a larger unit cell than amorphous silica.
(e) The atoms in amorphous silica do not pack as efficiently in three dimensions as compared to the atoms in quartz.