请教一道语法题目

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 

答案c，请问，是不是因为后面有“the world has ever seen”这一句，所以前面也要用完成时？如果没有这个限定语，直接用believed to be就可以了？

比如说，猎豹被认为是人们见过的陆地上奔跑最快的动物。这里是用to be呢，还是to have been？

谢谢解答！

猎豹的例子要用to be.

主要参考贴：

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=91380

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=23279

其他参考帖子：

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=29891

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=22920

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=15657

大全529， OG11th 42

Quasars, at billions of
light-years from Earth the most distant observable objects in the universe, believed
to be the cores of galaxies in an early stage of development.

(A) believed to be

(B) are believed
to be

(C) some believe
them to be

(D) some believe
they are（B）

(E) it is believed
that they are

OA: B 虽然考点不在have上， 但是可以看到ETS承认believed
to be