题目

I guessed that Mother some sandwiches by the time we got back. A . would make B . would have made C . have made D . made 答案：B 声呐是人们利用超声波制成的一种仪器，我们可以利用声呐来探测鱼群、潜艇等．已知超声波在海水中的传播速度是1450米/秒，若将超声波垂直向海底发射信号，经过4秒钟后收到反射回来的声波，则海洋深度是________m．