A、The increased pressure. B、Breathing. C、The coldness and the darkness. D、Dangerous sea lives. B

Everyone enjoys the seaside. Beaches are ideal places for games, sunbathing and swimming. A warm sun, a clean sea and a sandy shore are most people’s idea of the perfect holiday setting. But beyond these sunny, warm, shallow shorelines lie the cold depths of the sea. Here below the surface, where the sun’s rays and light quickly fade away, is a place as hostile to men as the black depths of space. Every 33 feet below the surface the pressure on every inch of human body increases by 1 atmosphere. Down to 33 feet the body is subjected to the same pressure we endure in the upper world without even noticing it—14.7 pounds per square inch. At 600 feet down in the sea the pressure exerted by that great volume of water is over 280 pounds per square inch. The wonderful thing is that the human body can survive such pressure without ill effects. But pressure is not the only problem. There is also the cold and the darkness. As you have just heard, the sun’s rays only reach a short way down into the sea. The water temperature falls and the light dims to complete blackness as you go lower down.
    The sea is not an empty space—it is full of life, some forms of which are dangerous to men. Of course the greatest problem of all is breathing. Every breath of air you take must be pumped to you from above or carried with you in tanks. You must be extremely careful when you come up again from the depths in order to avoid the painful and often deadly disease known as "the bends". When a diver only breathes compressed air, nitrogen builds up in his blood stream. If he comes up too quickly to the surface the nitrogen fizzes, like lemonade when the top is taken off the bottle. Finally there is the constant danger of faulty equipment: An accident 200 feet below the surface is very likely to be fatal.