A. quite B. mountainous C. disadvantages D.by E. deserved F. compare G. range H. with I. developed J. flat K. equal L. earned M. failure N. generated O. for Our ape-men forefathers had no obvious natural weapons in the struggle(1)______survival 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 provided an impressive “small fire” weaponry . They could not even defend themselves (3)______ 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 doomed to (4)______ and extinction. But they were endowed with enormous concealed advantages of a kind not possessed by any of their competitors.  In the search for the pickings of the forest, the ape-men had (5)______ efficient stereoscopic vision and a sense of colour 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 beyond the reach of the original inhabitants of the grassland. Good long-distance sight was (6)______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 (7)______ of sight — so what they had to do was climb a tree. Out in the open, however, this simple solution was not available. Climbing a hill would have helped, but in many places the ground was (8)______.The ape-men adopted the only possible solution. They reared up as high as possible on their hind limbs and began to walk upright.  This vital change of physical position brought about considerable (9)______.It was extremely unstable and it meant that the already slow ape-men became slower still. However, they persevered and their bone structure gradually became adapted to the new, unstable position that (10) ______ them the name Homo Erectus, upright man.