19.The students didn’t manage to work out the problem ______ their teacher had explained how.
问题描述:
19.The students didn’t manage to work out the problem ______ their teacher had explained how.
答
应该是until
not until,直到.....才
答
before
答
until
答
unless
主句是过去式,从句是过去完成时,那从句的动作必须在主句之前,而非之后