单项选择题

Several fishermen in Maine recently completed a study program at the country’s first ever "Cod(金枪鱼)School." It trains fishermen who usually earn a living fishing in the ocean to be fish farmers. The program is aimed at helping commercial fishers to find a new way to carry out their trade.
On a recent morning, a fishing boat left, but the men on the boat were not going fishing, they were going farming. They are going to move the cages and sorting codfish. About one and a half kilometers out to sea, the boat finds eight circular pens. A rubber tube encloses each one. The pens are covered with netting material to keep out seabirds. Inside each of the fifty-meter wide areas are up to fifty thousand cod. Most of these fish will be served on dinner tables around the world.
The program will teach the students how to feed the fish and not to over-feed the fish. It means that they should give them enough feed, and not waste any feed and make it as efficient as possible.
The fish-farmers in training take turns throwing special fish food into the pen. Air bubbles appear as thousands of cod come up to feed. They can be seen from the boat with an underwater camera.
Bill Thompson is one of the Cod School’s four students. He says the program has showed him that fish-farming is a wise choice. He said: "Even if the wild stocks came back to their fullest capacity they still wouldn’t feed the world. So this is the way of the future. And it’s possible for a family to run a business also.\

In the "Cod School" cod fish are kept in ().

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