Covalent bonding occurs in both molecular and covalent-network solids. Which of the following statements best explains why these two kinds of solids differ so greatly in their hardness and melting points?
(a) The molecules in molecular solids have stronger covalent bonding than covalent-network solids do.
(b) The molecules in molecular solids are held together by weak intermolecular interactions.
(c) The atoms in covalent-network solids are more polarizable than those in molecular solids.
(d) Molecular solids are denser than covalent-network solids.