<p>According to the answer, the correct sentence is:
The speed of sound in water is greater than [that of sound] in the air.
Then, is it also right to say "The speed of sound in water is greater than [that] in the air?</p>
<p>No, " than that in air" would not be correct. (Don’t trust me fully, by the way, because although i get 770+ in writing, i usually get one or two of these questions wrong) “That in air” is still too ambiguous.</p>
<p>Yeah, ambiguity is the problem. Nothing formal, as far as I can tell.</p>
<p>Try simplifying — Dancing in the dark is better than that in the day.</p>
<p>Just…meh.</p>
<p>Thx!!!</p>
<p>So “that” can only referred to one word, right?</p>