2012年大學(xué)英語四級<完形填空>模擬試題28

　　Directions: There are 20 blanks in the following passage. For each blank there are four choices marked [A], [B], [ C] and [ D ] on the right side of the paper. You should choose the ONE that best fits into the passage. Then mark the corresponding letter on Answer Sheet 2 with a single line through the centre.
　　注意：此部分試題請在答題卡2上作答。
　　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 strong 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 __7_4__ 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? 中 華 考 試 網(wǎng)
　　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.