题目

一 When did the computer crash?一 This morning, while Ithe reading materials downloaded from some websites. A . have sorted B . was sorting C . am sorting D . had sorted 答案：B书架上有两套同样的教材，每套分上、下两册，在这四册教材中随机抽取两册，恰好组成一套教材的概率是 [     ]A.B.C.D.