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
the sentence is stating about Bering land bridge, which got disappeared due to environmental changes BUT people still
consider that human being beings first entered Americas over that bridge.
"were " makes the sentence as if people used to think that the humans entered Americas over bridge but NOT NOW.
so, "are thought" will be used because THOUGH the bridge is no more BUT people still think that the bridge was first used by humans to enter Americas.
" Make more efforts "
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