A student wrote in saying, "The correct answer states that the ratio of 'covalently-bonded hydrogens to lone pairs of electrons on the oxygen atom..' lone pairs are not involved in covalent bonds, they are involved in hydrogen bonds."
While it is correct that lone pairs are involved in hydrogen bonds and not covalent bonds, this answer choice is stating that it's the ratio of electron lone pairs to covalently-bonded hydrogens that is important in water's intermolecular forces. In other words, because there are two lone pairs and two covalently-bonded hydrogens, the amount of hydrogen bond interaction can be large between molecules of water. There are two hydrogen bond donors and two hydrogen bond acceptors. Because the ratio is equal (and because of the bent structure), solid water has peculiar characteristics--namely, being less dense than liquid water.
Hope this helps!
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