 Notice, however, that whereas in (30a) the propositions in \(X\) other than \(P\) are false, in (30b) all three members of \(X\) are true: birds, mammals, and reptiles reproduce. This, Cross argues, reflects the fact that how-questions can exhibit two different kinds of explanatory contrast. By asking (30a) one requests an answer that highlights those special qualities of DNA that enable it to replicate and that benzene and hexane do not possess. By asking (30b), on the other hand, one requests an answer that highlights the differences between the way in which reptiles reproduce and the ways in which mammals and birds reproduce. The latter kind of explanatory contrast also appears to be at play when (30b) is re-worded this way: