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.

应该是until
not until,直到.....才

before

until

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