从甲地到乙地有一段上坡与一段平路．如果保持上坡每小时走3km，平路每小时走4km，下坡每小时走5km，那么从甲地到乙地需54min，从乙地到甲地需42min．设从甲地到乙地上坡与平路分别为xkm，ykm，依题意，所列方程组正确的是（　　）

材料一 改革开放至今的城乡居民收入比：1978年这一数据是2.56倍，其后随着农村率先进行改革，到1983年这一数据将是1.82倍。二十世纪九十年代初期城乡居民收入比总体变化不大，但到了1999年，随着重工业增速超过轻工业并引领工业化进入中后期，这一数据明显攀升。2012年我国城乡居民人均可支配收入为24565元，农村居民人均纯收入为7917元，二者之比为3.10倍。

材料二 目前我国社会总体上是和谐的。但是，也存在着不少影响社会和谐的矛盾和问题，主要是城乡、去、经济社会发展很不平衡……就业、社会保障、收入分配等关系群众切身利益的问题比较突出。因此，要加强制度建设，保障社会公平正义，完善收入分配制度，规范收入分配秩序，在经济发展的基础上，更加注重社会公平。

    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."