单选题 The students didn't manage to work out the problem ______ their teacher had explained how.
  • A. when
  • B. until
  • C. unless
  • D. as
【正确答案】 B
【答案解析】[解析] 根据题意，本题考查not...until的用法。所以答案为B。译文：直到老师讲解，那些学生才解出了那道题。