轮船沿江从A港顺流行驶到B港，比从B港返回A港少用3小时，若船速为26千米/时，水速为2千米/时，求A港和B港相距多少千米．设A港和B港相距x千米．根据题意，可列出的方程是（　　）

     Australia is nearly as large as the United States, but most of it is too dry to live in. Around the edge(边缘) of this huge dry part are large sheep and cattle farms. A few of them are as large as the smallest states in America. Often the nearest neighbors are many hundred miles away.

     The two-way radio is very important to people who live on these great Australian farms. It works much like a telephone. A person can listen to someone else talk and give an answer.

     When these radios first came into use, the Australian government set up a special(专门的，特别的) two-way radio programme(节目). Then, people on the large farms could talk to a doctor hundreds of miles away. They could tell the doctor about someone who was sick, and the doctor could let them know how to care for the sick person.

     Since the large farms were so far from towns, the children could not to go to school. Radio schools were set up for them in some places. At a certain time each day, the boys and girls turn on their radios and listen to teachers in cities miles away.

     Families on the large farms wanted to give news to their neighbors. “Round robin(知更鸟)” talks by radio were started to keep families in touch with each other. They could talk about who was getting married or who was sick or who was going away. The men could talk about their sheep and cattle and how much money the markets would pay for them. In many ways the radio became a newspaper for the farm people of Australia.