[求助]大全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

请问，为什么用have entered，难道enter这个动作持续到现在？可是那个bridge很早就消失了呀。

谢谢！

顶一下。

我也很迷惑为什么要用完成时态呢。这里为什么用are thought二而不用were thought呢。