Rivers are one of our most important natural resources. Many of the world’s great cities

Rivers are one of our most important natural resources. Many of the world’s great cities are located on rivers, and almost     1     country has at least one river     2    (flow) through it that plays     3     important part in the lives of its people.
Since the beginning of history, people _4     (use) rivers for transportation. The longest one in the United States is the Mississippi, and the lifeline of Egypt is the Nile.     5     transportation, rivers give water to drink, water for crops, and chances for fun and recreation （消遣，娱乐活动）for the people     6     live along their banks.
However, large cities and industries that are located on rivers often make problems. As the cities grow     7     size and industries increase in number, the water in the rivers bees     8     (pollute) with chemicals and other materials. People e to kwon the     9     (important) of doing more to keep their rivers clean     10     they want to enjoy the benefits of this natural resource.    1. every / each         2. flowing    3 an       4 have used    5 Besides