题目

 --- I can’t find Xiao Ping. Where did you see him this morning? --- In the reading room _________ he often studies in. A. that             B. where            C. in which             D. the one 答案：A一架飞机水平匀速地在某同学头顶飞过，当他听到飞机的发动机声从头顶正上方传过来时，发现飞机在他前上方约于地面成60°角的方向上，据此估计出飞机的速度约为声速的         倍。