55. A cost         B spend       C take       D pay for

Rivers are one of our most important natural resources(自然资源). Many of the world’s great cities are located on rivers, and almost every country has at least one river flowing through it that plays an the lives of its people.

Since the beginning of history, people have used rivers for transportation. The longest one in the United States is the Mississippi. The lifeline of Egypt is the Nile. To the people of India , the Ganges is great, but it is also important for transportation; ships can travel along it for a thousand miles. Other great rivers are the Congo in Africa and the Mekong in southeast Asia, The greatest of all for navigation(航海), however, is the Amazon in Brail, It is so wide and so deep that large ships can go about two thousand miles upon it.

Besides transportation, rivers give food, water to drink, water for crops, and chances for fun and recreation for the people who live along their banks. In order to increase the supply of water for crops, engineers sometimes build a dam(大坝) across a river and let a lake form behind the dam. Then people can use the water not only to irrigate(灌溉) their fields but also to make electricity for their homes and industries.

However, large cities and industries that are located upon rivers often make problems. As the cities grow in size and industries increase in number, the water in the rivers becomes polluter with chemicals and other materials. People are learning the importance, however, of doing more to keep their rivers clean if they want to enjoy the benefits(利益) of this natural resource.

54. A When       B While       C Until      D Since

53.A swap        B change      C exist       D influence

52.A be corrected   B be copied    C be read     D be written

51. A find out      B carry on     C work out     D work on

50.A map         B plan        C decision     D discussion

49.A excited       B experienced  C interesting     D extra

48. A mysterious    B surprised    C pleased     D pleasant