If they don't understand it the first time, over it again until they do． [ ] A．g

If they don't understand it the first time, ________ over it again until they do．

A．going

B．to go

C．gone

D．go

试题答案

答案：D

横线前if引导的是一个条件状语从句，而在横线处无论是填A、B或C项，都是一个非谓语动词短语，而不是一个句子，这样一来，整个句子没有主句，从而导致结构不完整。若选D项，则go over it again until they do是一个句子(祈使句)，在此用作整个复合句的主句。