Rivers are one of our most important natural resources  . Many of the world's great cities are located  on rivers, and almost every country has at least one river running through it that plays an important role in the lives of its people. Since the beginning of the 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 Gangs 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  , however, is the Amazon in Brazil. 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 entertainment for the people who live along their banks. In order to increase  the supply of crops, engineers sometimes build a dam   across a river and let a lake from behind the dam. Then people can use the water not only to irrigate   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 polluted 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   of this natural resource. The greatest river for navigation is   _  .
the Amazon the Nile the Mekong the Mississippi