Earth is believed ______ by an object the size of Mars at some point in the distant past before itssurface cooled.
A．to be hitB．to have hit
C．to have been hit D．to hit
C

試題分析：句意：人們相信，地球在其表面冷卻之前被類似火星大小的物體撞擊過。分析語境可知主語Earth和hit之間為動賓關(guān)系，所以用被動語態(tài)。“被撞擊”be hit這個動作發(fā)生在“冷卻”cool這個動作之前，故用完成時態(tài)。故選C。
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