The new employees had to go over the safety regulations several times____ they understood them.

题目

The new employees had to go over the safety regulations several times____ they understood them.
A.since B.before C.unless D.when
我怎么感觉b和c意思上都对啊

答案

答案为B
译为：新来的员工必须多次浏览安全守则,直到他们彻底理解了.
如果要填unless 则应改为unless they had understood them.