阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中词语的正确形式填空，并将答案填写在答题卡标号为31－40的相应位置上。

Rivers are one of our most important natural resources. Many of the world’s great ____小题1:____ are located on rivers, and almost ___小题2:____ country has at least one river flowing through it that plays ____小题3:____ important part in the lives of its people.

Since the beginning of history, people _____小题4:_____ (use) rivers for transportation. The longest one in the United States is the Mississippi, and the lifeline of Egypt is the Nile. _____小题5:_____ transportation, rivers give water to drink, water for crops, and chances for fun and recreation for the people ___小题6:___ live along their banks.

However, large cities and industries that are located on rivers often make problems. As the cities grow __小题7:____ size and industries increase in number, the water in the rivers becomes ____小题8:_____ (pollute) with chemicals and other materials. People are learning the ___小题9:_____ (important) of doing more to keep their rivers clean ___小题10:_____ they want to enjoy the benefits of this natural resource.