Our ape-men forefathers had no obvious natural weapons in the struggle for survival in the open. They had neither the powerful t

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问题     Our ape-men forefathers had no obvious natural weapons in the struggle for survival in the open. They had neither the powerful teeth nor the strong claws of the big cats. They could not【C1】______with the bear, whose strength, speed and claws【C2】______an impressive " small fire" weaponry. They could not even【C3】______themselves by running swiftly like the horses, zebras(斑马)or small animals. If the ape-men had【C4】______to compete on those terms in the open, they would have been【C5】______to failure and extinction. But they were【C6】______with a great deal of【C7】______advantages of a kind not possessed by any of their competitors. In the search【C8】______the pickings of the forest, the ape-men had【C9】______efficient stereoscopic(立体感的)vision and a sense of color that the animals of the grasslands did not possess. The ability to see clearly at close range permitted the ape-men to study practical problems in a way that lay far【C10】______the reaches of the original inhabitants of the grassland. Good long-distance sight was【C11】______another matter.
    Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the higher the viewpoint, the greater the range of sight—so【C12】______they had had to do was climb a tree. Out in the open, however, this simple solution was not【C13】______. Climbing a hill would have helped, but the ground was【C14】______in many places. The ape-men【C15】______the only possible solution. They【C16】______up as high as possible on their hind limbs and began to walk upright.
    This vital change of【C17】______position brought about considerable disadvantages. It was extremely unstable and it meant that the already slow ape-men became slower still.【C18】______, they persevered and their bone structure gradually became【C19】______to the new, unstable position that【C20】______them the name of upright man.
【C13】

选项 A、sufficient
B、enough
C、available
D、convenient

答案C

解析
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