【题目】It may be necessary to stop______ in the learning process and go back to the difficult points in the lessons.
A.at intervals
B.at random
C.at ease
D.at length

【答案】A
【解析】A．at intervals 不时的；B．at random随意的；C．at ease自在的；D．at length详尽地；句意：可能有必要在学习的过程中再停下来回顾一下本课的难点。根据语境，所以答案是A。