题目内容：
请根据短文内容，回答题。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 them--and, like all mammals, originated on land. There are many signs of this.
__________ (1) 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 whale's body is one gigantic muscle. The blue whale's pulling __________ (2) has been estimated at 400 horsepower. One specimen was reposed to have __________ (3) a whaling vessel for seven hours at the __________ (4) of eight knots.
An enraged whale will attack a ship. A famous__________(5) of this was the fate of whaler Essex, which was sunk__________(6) 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__________(7) the old-time whaleboats in their jaws and crush them.
The greatest mystery of whales is their diving ability. The sperm whale can __________(8) the bosom for his favorite food, the octopus (章鱼). __________(9) that search he is known to go as far down as 3,200 feet, where the pressure is 1,400 pounds to the square inch. Doing__________(10) he will remain submerged (水下的 ) as long as one hour. Two feats (绝技) are involved in this: storing up enough __________ (11) (all whales are air-breathed) and withstanding the great change in pressure. Just__________(12) 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__________ (13) 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 __________(14) you can't bring a live whale into the laboratory for study, no one __________(15) just how these things work.
_________ A．Their
B．It's
C．Theirs
D．Its
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