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An adult giraffe’s head is about six feet above its heart. This means that to pump enough blood up to the brain the circulatory system 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 unusually high blood pressure, about double that of human beings. But an international team of biologists began to wonder about this. If giraffes have such high blood pressure, they should have a terrible problem with swelling in their legs and feet. Why don’t giraffes have swollen feet
Giraffes should have another problem, too. Every time they bend heads down to drink, the blood should rush to their heads and have a hard time flowing back up (when the head is down)to the heart. How come giraffes don’t black out when they drink
The answer to the swollen feet problem, the researchers found, is that giraffes have what t
A. what physical features the giraffe has
B. how the giraffe’s heart works
C. how the giraffes heart pumps blood to its head
D. how the giraffes can stand up to high blood pressure

[单项选择]

An adult giraffe’s head is about six feet above its heart. This means that to pump enough blood up to the brain the circulatory system 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 unusually high blood pressure, about double that of human beings. But an international team of biologists began to wonder about this. If giraffes have such high blood pressure, they should have a terrible problem with swelling in their legs and feet. Why don’t giraffes have swollen feet
Giraffes should have another problem, too. Every time they bend heads down to drink, the blood should rush to their heads and have a hard time flowing back up (when the head is down)to the heart. How come giraffes don’t black out when they drink
The answer to the swollen feet problem, the researchers found, is that giraffes have what t
A. their legs and feet have strong skin and muscles
B. their blood vessels are stiff and tough legs
C. they wear a kind of special suit
D. there are lots of one-way valves in the veins of their legs

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