题目：
| An adult giraffe’s head is about six feet above its heart. This means that to (67) enough blood up to the brain the circulatory (68) must be p enough to keep the blood at very high pressure. Biologists have known for some time that giraffes solve this problem by having (69) high blood pressure, about (70) that of human beings. But an international team of biologists began to (71) about this. If giraffes have such high blood pressure, they should have a (72) problem with swelling in their legs and feet. Why don’t giraffes have swollen feet Giraffes should have (73) problem, too. Every time they bend heads (74) to drink, the blood should (75) to their heads and have a hard time (76) back up (when the head is down) to the heart. How come giraffes don’t black out when they drink The answer to the (77) feet problem, the researchers found, is that giraffes have (78) the researchers call a "natural anti-gravity suit". It (79) out that the skin and other (80) in their legs and feet are (81) stiffer and tougher than those of other (82) . 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 (83) the giraffe bends down to drink The researchers found that the giraffe’s jugular vein, which (84) 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 and relax repeatedly as the animal moves its head and sucks (85) drinking water. By squeezing the valved jugular vein, they (86) blood moving back to the heart even while the animal is drinking. |
A．what
B．where
C．that
D．those
答案：
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参考答案：平行
