daquan 325

1.        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（A）

(E) were thought to first enter

why use "have entered"? can enter (the istant action) be used as have entered?

help me please!

when human beings  are thought to 的时候,human beings  已经entered the Americans,也就是说entered the Americans发生在前,而  are thought to 发生在后,应该用现在完成时态