Man wants to explore deep into the sea.However, he is not a fish.He must breathe air, so he can't stay under the water's surface for any length of time.To explore deep water, man faces even more dangers and problems.

　　A diver who wants to stay under water for more than a few minutes must breathe air or a special mixture of gases.He can wear a diving suit and have air pumped to him from above.He can carry a tank of air on his back and breathe through a nose and a mouthpiece.Water weighs 800 times as much as air.Tons of water pushes against a diver deep in the sea.His body is under great pressure.

　　When a diver is under great pressure, his blood takes in some of the gases he breathes.As he rises to surface, the water pressure becomes less.If the diver rises too quickly, the gases in his blood form bubbles.The diver is then suffering from the bends.The bends can cause a diver to double up in pain.They can even kill him.

　　20．On the whole, this passage is about ________.

　　A．special mixtures of gases in deep sea

　　B．the problems man faces in deep­sea diving

　　C．air pressure under the surface of sea water

　　D．a kind of illness that man suffers in the sea

　　21．The passage does not say so, but it makes you think that ________.

　　A．deep­divers should be in good health

　　B．divers explore the deep sea only for treasure

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