Only in this way, directed by the teacher, work out the problem the day before.
A. had they
B. they could
C. could they
D. did they
网友回答
A. had they
B. they could
C. could they
(答案→)D. did they
解析：考查倒装句。当only位于句首是，主句采用部分倒装，助动词提前。句意：只有按照老师指导的这种方法，前一天他们才能解决这个难题。结合语境可知本句描述的是过去的动作，故用一般过去时态。助动词用did，选D。