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：（A）

疑惑1）为什么用现在完成时态

        2）WHO caused the sea level to rise several hundred feet worldwide，感觉massive glaciers似乎不对，因该是massive glaciers melted 这件事情？