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【C1】______ of the big cats. They could not【C2】______ with the bear, whose strength, speed and claws【C3】______ an impressive "small-fire" weaponry. They could not even defend themselves【C4】______ running swiftly like the horses, zebras or small animals. If the ape-men had attempted to compete on those terms in the open, they would have been【C5】______ to failure and extinction. But they were【C6】______ with enormous concealed advantages of a kind not possessed by any of their competitors.

In the search 【C7】______ the pickings of the forest, the ape-men had【C8】______ efficient stereoscopic vision and a sense of colour that the animals of the grasslands did not possess. The ability to see clearly at close 【C9】______ permitted the ape-men to study practical problems in a way that lay far【C10】______ the reach 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 to do was climb a tree. Out in the open, however, this simple solution was not【C13】______ Climbing a hill would have helped, but in many places the ground was flat. The ape-men【C14】______ the only possible solution. They reared up as high as possible on their【C15】______ limbs and began to walk upright.

This【C16】______ change of physical position brought about considerable【C17】______ .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 Homo erectus, upright man.

【C1】

A．claws

B．paws

C．hoofs

D．hands