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 ________ are located on rivers, and almost _______ country has at least one river flowing through it that plays ________ important part in the lives of its people.
Since the beginning of history, people __________ (use) rivers for transportation. The longest one in the United States is the Mississippi, and the lifeline of Egypt is the Nile. __________ transportation, rivers give water to drink, water for crops, and chances for fun and recreation for the people ______ live along their banks.
However, large cities and industries that are located on rivers often make problems. As the cities grow ______ size and industries increase in number, the water in the rivers becomes _________ (pollute) with chemicals and other materials. People are learning the ________ (important) of doing more to keep their rivers clean ________ they want to enjoy the benefits of this natural resource.