Between C and D:
C has an 'it' which does not have a clear antecedent. In other words what does 'it' refer to?
"discovery", "new class" or "corn".. ---Thus Ambiguous..
Always be wary of 'it'
the issue is not ambiguity -- remember that ambiguity is NOT a reliable criterion on which to eliminate.
(see here: post40400.html#p40400
rather, the problem is that the pronoun in this case is just flat-out incorrect
-- neither "new class" or "corn" makes sense
if substituted for the pronoun. remember, the sentence has to make LITERAL SENSE if you SUBSTITUTE the antecedent into the location of the pronoun.
if you ask yourself, VERY LITERALLY, "exactly what
led to this greater understanding?", then you'll find that the answer is neither the new class nor the corn -- both of those are inanimate objects, which by themselves can't contribute to human understanding.
thing that led to this greater understanding was the discovery
described in the sentence; hence the wording of the correct answer.
if you didn't like the phrasing of the correct answer, then you probably aren't as comfortable as you should be with ABSTRACT NOUN APPOSITIVES.
for a short description, with examples, see here:http://www.beatthegmat.com/sc-with-surf ... tml#209928