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第6部分：完形填空(第51～65题，每题l分，共15分) 下面的短文有l5处空白，请根据短文内容为每处空白确定1个最佳选项。 The Greatest Mystery of Whales The whale is a mammal(哺乳动物)- warm-blooded, air-breathing, giving birth to its young alive, sucking themand, like all mammals, Originated on land. There are many signs of this. (51)front flippers(鳍肢), used for steering and stability, are traces of feet. Immense strength is built into the great body of the big whales, and in fact most of a whales body is one gigantic muscle. The blue whales pulling (52)has been estimated at 400 horsepower. One specimen was reposed to have (53)a whaling vessel for seven hours at the (54)of eight knots. . An enraged whale will attack a ship. A famous (55)of this was the fate of whaler Essex, which was sunk (56)the coast of South America early in the last century. More recently, steel ships have had their plates buckled(扭曲变形)in the same way. Sperm whales were known to (57)the oldtime whaleboats in their jawsand crush them。 The greatest mystery of whales is their diving ability. The sperm whale can (58)the bosom for his favorite food, the octopus(章鱼). (59)that search he is known to go as far down as 3,200 feet, where the pressure is l,400 pounds. to the square inch. Doing (60)he will remain submerged(水下的)as long as one hour. Two feats(绝技)are involved in this：storing up enough (61)(all whales are airbreathed)and withstanding the great change in pressure. Just (62)he does it scientists have not determined. It is believed that some of the oxygen is stored in a special system of blood vessels, rather than just held in the lungs. And (63)is believed that a special kind of oil in his head is some sort of compensating mechanism that automatically adjusts the internal pressure of his body. But (64)you cant bring a live whale into the laboratory for study, no one____(65)just how these things work. | 
