Question

The students didn’t manage to work out the problem ________ their teacher had explained how．

1. A.
   when
2. B.
   until
3. C.
   unless
4. D.
   as

Answer 1

C
從句中動(dòng)詞的時(shí)態(tài)為過去完成時(shí)，可知其動(dòng)作發(fā)生在主句謂語動(dòng)詞之前。最大的干擾項(xiàng)是B項(xiàng)，但until引導(dǎo)時(shí)間狀語從句時(shí)，從句的動(dòng)作常常發(fā)生在主句動(dòng)作之后，因此，本句時(shí)態(tài)搭配不合理；unless＝if not。句意為：學(xué)生們不能解決這個(gè)問題，除非老師事先解釋過了。