请教大全补充207题

                             Geologists believe that the Bering land bridge, over which human beings are thought to have first entered the Americans, disappeared about 14,000 years ago when massive glaciers melted and caused the sea level to rise several hundred feet worldwide.

(A) are thought to have first entered

(B) were thought first to enter

(C) were thought at first to enter

(D) are thought of as first entering

(E) were thought to first enter

答案是A。 我的问题是E的错误在哪儿？ 是不是因为主句用现在时，所以这里也用现在时？ 另外在thought to 后面用完成时的依据是什么？ 谢谢。