GMAT Sentence Correction (SC) | Re: A piece pf limestone discovered

abhi398 had the right idea, but he expressed it in unconventional language so later commenters seemed to have misunderstood his point. I'll clarify.

The original question was, "How could...extinction may have been caused... be correct, sinceextinction is singular andhave is plural?"

The answer is that the so-called modal verbs used to express past possibility and probability sometimes take different forms than do other verbs. For instance, these are all correct,
He could have