1.Raymond entered college at the age of eighteen,_a__four years later at the head of his class.因为两句中间是一逗号,没有连词,不能用谓语形式.只能用现在分词这种非谓语,
A.graduating B.graduated C.to graduate D.having graduated
2.I was busy the other day,otherwise I _ A.__to help you.
A.would have come B.would come C.had come D.came
表示过去的虚拟,从前一句上看出时间是过去.所以后一句是说过去就可以做.但没做.相当于 if i had not been busy the other day,i would have come to help you.
3.we put our books in the bookcase to keep them _ B__from dust.
A.empty B.free C.loose D.short
keep ...free from 是固定搭配.