选择题 All of them wondered when-------- A. they can set off B. can _英语解答

选择题 All of them wondered when-------- A. they can set off B. can they set off C.to set off
答案给的是C，when to set off。谁能给我解释一下

人气：452 ℃　时间：2020-01-25 19:06:40

解答

C
如果选A,可以改为 they could set off
因为主句的动词是过去式wondered ,从句时态要与之保持一致.能解释一下这是什么考点吗？