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  3. The largest animal that ever lived on land or in water still exists. Not even the giant dinosaurs were as large as some whales.

The largest animal that ever lived on land or in water still exists. Not even the giant dinosaurs were as large as some whales.

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问题     The largest animal that ever lived on land or in water still exists. Not even the giant dinosaurs were as large as some whales. One sulphur-bottom whale caught in the Antarctic was 110 feet long, and weighed between 90 and 100 tons.
    Whales can grow to such enormous size because their bodies are supported by the water. An animal that lives on land can only grow to a size that his legs can support, while a bird’s size is limited by its wing size. A whale has none of these difficulties.
    Millions of years ago, whales lived on land and walked on four legs. Today, whales still have small bones that are the remains of their hind legs, but these bones can only be seen on the inside of the whale. No one knows why whales left the land to live in the water. However, scientists can surmise that when the whales changed their environment, their bodies underwent a change—taking on a more fish-like appearance. This new form offered less resistance to the water, enabling the whales to swim faster. Despite their fish-like form, whales are not fish. A whale will drown, just as a man will, if it stays under water too long. When a whale is under water, it closes its nostrils tightly and holds its breath. The air in its lungs becomes very hot and full of water vapour. When the whale rises to the surface and exhales, its hot breath produces a column of water vapor that rises high in the air. A man produces the same effect when he exhales warm air on a cold morning.
    Whales are classified as mammals because they bear their young, rather than laying eggs, and because the mother whales give the babies milk. Like other mammals, whales have warm blood. Their blood stays at the same temperature, even when they move from hot to cold water. They keep warm in cold water because they have a thick layer of fat just under their skins. This fat is called blubber, and it is thicker on whales that spend their lives in cold water. Almost all land mammals, except man, have hair on their bodies to keep them warm, but whales, which have very few hairs, are kept warm by their fat.
    Whales do not bear young more often than every two years. The births are usually single births, but there have been instances of whale twins. Mother whales show a great deal of affection for their young. If the baby whale is killed, the mother will stay close to it for a long time. The young grow very rapidly during their first three or four years. And, although no one is certain how long a whale lives, the normal life span is probably less than 100 years.
According to the passage, the blood temperature of a whale______.
选项 A、varies in hot water
B、remains the same
C、grows colder as the blubber accumulates
D、grows warmer as the whale grows older
答案B
解析
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