One of the earliest known birds with a beak and contour feathers, Confuciusornis sanctus, with large clawlike "thumbs" on its wings, which probably helped them to climb up to a launching position for flight.

A. with large clawlike "thumbs" on its wings, which probably helped them to
B. with large clawlike "thumbs" on their wings, which probably helped it to
C. had large clawlike "thumbs" on its wings, which probably helped them
D. had large clawlike "thumbs" on its wings, probably to help it
E. had large clawlike "thumbs" on their wings, probably to help it

Pay attention to the different pronouns in the answer choices. We can choose "on its wings" or "on their wings." The pronoun's antecedent is 'Confuciusornis sanctus,' which is singular, so we want "on its wings." Eliminate B and E. Next, we have another pronoun split "helped them" or "helped it." The pronoun is still referring to Confuciusornis sanctus, so again, we want "it." Eliminate A and C. All that's left is D, which is the answer.