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  3. An adult giraffe’s head is about six feet above its heart. This means that to【C1】______enough blood up to the brain the circulat

An adult giraffe’s head is about six feet above its heart. This means that to【C1】______enough blood up to the brain the circulat

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问题     An adult giraffe’s head is about six feet above its heart. This means that to【C1】______enough blood up to the brain the circulatory【C2】______must be strong enough to keep the blood at very high pressure.
    Biologists have known for some time that giraffes solve this problem by having【C3】______high blood pressure, about【C4】______that of human beings. But an international team of biologists began to【C5】______about this. If giraffes have such high blood pressure, they should have a【C6】______problem with swelling in their legs and feet. Why don’t giraffes have swollen feet?
    Giraffes should have【C7】______problem, too. Every time they bend heads【C8】______to drink, the blood should【C9】______to their heads and have a hard time【C10】______back up(when the head is down)to the heart. How come giraffes don’t black out(晕倒)when they drink?
    The answer to the【C11】______feet problem, the researchers found , is that giraffes have【C12】______the researchers call a "natural anti-gravity suit". It【C13】______out that the skin and other【C14】______in their legs and feet are【C15】______stiffer and tougher than those of other【C16】______. As a result, the blood vessels in the leg cannot swell.
    Therefore, the blood has nowhere to go but back to the heart. What about blood rushing to the head【C17】______the giraffe bends down to drink? The researchers found that the giraffe’s jugular(颈静脉的)vein, which【C18】______blood from the head back to the heart, has lots of one-way valves in it. In the giraffe’s neck, there are lots of muscles that flex(曲折；收缩)and relax repeatedly as the animal moves its head and【C19】______up drinking water. By squeezing the valved jugular vein, they【C20】______blood moving back to the heart even while the animal is drinking.
【C20】
选项 A、permit
B、prevent
C、retain
D、keep
答案D
解析 空格所在句意为：通过挤压劲静脉的阀门，它们能______血液流回到心脏，甚至在长颈鹿喝水的时候也能。keep符合句意，所以D正确。
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