单项选择题

The floors of the ocean have many riches that can be used by man. Oil and chemicals and minerals (矿物) already are taken from the sea. By using nuclear energy, ocean water can be turned into fresh water by taking the salt out. In the near future, it is possible for human beings to produce food from farms under the sea. Food grown in the sea could help to meet the needs of population all over the world which is increasing fast. Thousands of people go hungry every day. About ten to fifteen percent of the world’s people do not have enough food. Some scientists believe that someday the sea will be used to make electric power. This would help to meet the need for more power for the world’s industries. The decreasing supply of coal, oil and gas shows that the need to find new kinds of power is urgent.
______ can be used to turn salt water into fresh water.