The earth is believed____ by an object the size of Mars at some point in the past before it's surface cooled down
问题描述：
The earth is believed____ by an object the size of Mars at some point in the past before it's surface cooled down
A to be hit
B to have hit
C to have been hit
D to hit
答
选C 首先被撞,应该用被动语态,排除B和D,又因为有‘’In the past before ...''在过去的某一时间段之前,所以用完成时,排除A意为''地球被认为将会受到火星大小的物体在某点被撞击,在它的表面冷却之前的一段时间里'' 故选C