题目内容：
阅读下文，回答题。The Greatest Mystery of Whales
The whale is a mammal - 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, 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 reported 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__ South America early in the last century. More recently, steel ships have had their plates buckled 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 __8__ the bottom for his favourite food, the octopus. __9__ 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 as long as one hour. Two feats are involved in this : storing up enough __11__(all whales are air-breatheD． and withstanding the great change in pressure. Just __12__ he dose 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__ 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.
请在1处填写正确答案。 A．Their
B．His
C．Theirs
D．Its
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