科学工作者为了探测海底某处的深度，向海底垂直发射超声波，经过4s收到回波信号，海洋中该处的深度是多少？（声音在海水中的传播速度是1500m/s）

“地道战嘿地道战，埋伏下神兵千百万，千里大平原展开了游击战，村与村户与户地道连成片，侵略者他敢来，打得他魂飞胆也颤，侵略者他敢来，打得他人仰马也翻，全民皆兵，全民参战，把侵略者彻底消灭完……”，歌曲生动地体现了中共的全面抗战路线。制定该路线的是

    An old porter (搬运工人) had worked for the railway for a very long time. One morning he was standing in one of the big railway stations and waiting for travelers to ask him to help them with their luggage, when he saw a small man running toward the trains with a big bag in his hand.

    The porter watched the man for a few seconds, and the man saw the porter.

At once the small man ran up the porter and said, "Can I catch the 10：35 train to London, porter?" He was breathing very fast, and he seemed worried.

    The old porter looked at him for a moment and then said with a smile, "Well, sir, I'd like to help you, but I can't answer your question because I don't know how fast you can run along rails. You see," he explained, "the 10: 35 train to London left five minutes ago."