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fish farming is only one of the many recent experiments man has begun in the endless battle to feed himself. It is not as easy as it (18) ________. One doesn't simply find a pond, stock it with fish and wait for them to multiply. I knew one amateur who did just this. He looked on his pond as a fishing hole (19) ________ his friends. One morning he awoke to find its surface covered with dead fish. They had died quite naturally - from suffocation.

Commercially (20) ________ fresh-water fish need just that - and plenty of it. A good stream feeding the pond is one way to ensure that fish will stay healthy. Not only does it supply life-giving oxygen, it also helps to clear away the fish's own waste matter. With the oceans being overfished in recent decades, fish farming may soon take over there too.

18. A. sounds B. hears C. listens D. feels
19. A. with B. from C. kept D. raised
20. A. fed B. made C. for D. of