Our ape-men forefathers had no obvious natural weapons in the struggle for【1】in the open. They had neither the powerful teeth nor the strong claws of the big cats. They could not【2】with the bear, whose strength, speed and claws【3】an impressive " small fire" weaponry. They could not even defend themselves【4】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【5】to failure and extinction. But they were【6】with enormous concealed advantages of a kind not possessed by any of their competitors. In the search【7】the pickings of the forest, the ape-men had【8】efficient stereoscopic vision and a sense of color that the animals of the grasslands did not【9】. The ability to see clearly at close range permitted the ape-men to study practical problems in a way that lay far【10】the reach of the original inhabitants of the grassland. Good long-distance sight was【11】another matter. Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the【12】the viewpoint, the greater the range of sight-so【13】they had had to do was climb a tree. Out in the open, however, this simple solution was not【14】. Climbing a hill would have helped, but in many places the ground was flat. The ape-men【15】the only possible solution. They reared up as high as possible on their hind limbs and began to walk【16】. This vital change of physical position brought about considerable disadvantages. It was extremely unstable and it meant that the already slow ape-men became【17】still.【18】, they persevered and their bone structure gradually became【19】to the new, unstable position that【20】them the name Homo erectus, upright man.

A. survival
B. hunting
C. security
D. inhabitance