完形填空（共20小题；每小题1分，满分20分）
请认真阅读下面短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
Most people think of birds as feathered animals that fly. Scientists,
36 , do not define (给…下定义) birds as animals that fly, because some birds do not. Of the 10,000 or so species of birds, 46 cannot
37 . Flight plays a key role in
38 for most birds. It allows them to find food over a
39 area and to get away from enemies. Ancestors of flightless birds may have
40 their ability to fly because they had no regular predators (捕食者) or did not need to fly to find food. Rather than fly, some of these birds developed other
41 to catch food and avoid enemies. Two examples of
42 birds are penguins and ostriches.
43 birds that fly, penguins do not have wide wings or large feathers. In order to catch the fish they feed on, penguins use their powerful wings to swim
44 . And when they swim, they look as if they are flying through the water. The
45 of penguins’ bodies makes it possible for them to dive deep underwater,
46 their thick feathers protect them from the cold.
Ostriches are known for their long legs, long necks, and large size. To
47 themselves, they stay in groups and use their excellent sight and hearing to
48 enemies. As soon as
49 approaches, they can run at a speed of more than 65 kilometers per hour to
50 . In addition to using their strong legs to run, they can kick powerfully.
Not all flightless birds have been
51 in protecting themselves. Flightless birds on some islands had no enemies until people 52 . These birds were hunted and easily caught by people and the animals
53 by people. Human land development has
54 the habitats of some birds. A number of flightless birds
55 because they were unable to adapt to new conditions and new enemies.
| 小题1: | | A．thus | B．however | C．therefore | D．otherwise |
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| 小题3: | | A．growth | B．communication | C．health | D．survival |
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| 小题4: | | A．wide | B．familiar | C．crowded | D．special |
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| 小题5: | | A．developed | B．improved | C．lost | D．proved |
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| 小题6: | | A．parts | B．habits | C．ways | D．tools |
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| 小题7: | | A．wild | B．interesting | C．rare | D．flightless |
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| 小题8: | | A．Instead of | B．Because of | C．Besides | D．Unlike |
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| 小题9: | | A．quickly | B．carefully | C．differently | D．gradually |
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| 小题10: | | A．shape | B．color | C．bone | D．skin |
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| 小题12: | | A．help | B．feed | C．protect | D．hide |
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| 小题13: | | A．kill | B．notice | C．confuse | D．frighten |
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| 小题14: | | A．dawn | B．darkness | C．danger | D．food |
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| 小题15: | | A．look | B．escape | C．move | D．fight |
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| 小题16: | | A．successful | B．natural | C．unusual | D．positive |
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| 小题17: | | A．realized | B．acted | C．stopped | D．arrived |
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| 小题18: | | A．brought | B．found | C．bought | D．hunted |
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| 小题19: | | A．provided | B．formed | C．destroyed | D．controlled |
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| 小题20: | | A．flew away | B．watched out | C．gave away | D．died out |
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的电流方向和螺线管的 N、S 极． 
