【单选题】

From the beginning rivers have played an important part in the life of man. Man of the earliest times used the rivers as a means of traveling. Today rivers still serve as a great waterway for the transport and people.

In ancient times, man settled near rivers or on river banks and built up large empires.
Water is the Nature’s most precious gift to man. Man needs water to irrigate his crops, to cook and to wash. In nations all over the world rivers mean life and wealth. They feed and clothe the nations around them.
Water is also a source of energy and power. Man constructs huge dams across the river to control the water for irrigation and get the energy needed to drive generators. The electrical power is then directed to homes, cities, factories and television stations.
Man uses water each day. His main source of water comes from reservoirs, which in turn get their water from the rivers.
Rivers also bring down soil and minerals from the mountains and deposit them on the plains building up rich river deltas for raising plants and crops. Fresh water life in rivers or in lakes fed by them provide man with fooD、
In a small way rivers help to keep man in good health and provide for his amusements. Various forms of water sports keep man strong and healthy.
Rivers have run on this earth long before man. Man’s future ability to live is uncertain, but rivers will flow on forever.
Energy is obtained from rivers by______.A．building bridges across them
B．driving generators into them
C．building dams across them
D．directing them into homes and factories