Sentence Correction

A huge flying reptile that died out with the dinosaurs some 65 million years ago, the Quetzalcoatlus had a wingspan of 36 feet, believed to be the largest flying creature the world has ever seen.
(A) believed to be
(B) and that is believed to be
(C) and it is believed to have been
(D) which was, it is believed,
(E) which is believed to be

The correct answer is c. I discarded it because I thouhgt that "to have been" refers to "the Quetzalcoatlus" and then should be "has been" instead of "have been". Can anyone explain me why is have been correct?

I choose C because

a) wingspan is believed to be the largest flying creature
b) that is not appropriate. it refers to wingspan when it should refer to the bird
d & e) modifies wingspan

have and has - not clear in this