A．They built the pyramids with the help of ancient machines.
B．They built the pyramids with only human labors.
C．They built the pyramids with the help of aliens.
D．It is still unknown now.

Many prehistoric people subsisted as hunters and gatherers. Undoubtedly, gameanimals, including some very large species, provided major components of human diets.An important controversy centering on the question of human effects on prehistoric wildlifeconcerns the sudden disappearance of so many species of large animals at or near the end(5) of the Pleistocene epoch. Most paleontologists suspect that abrupt changes in climate ledto the mass extinctions. Others, however, have concluded that prehistoric people drovemany of those species to extinction through over-hunting. In their 'Pleistocene overkillhypothesis,' they cite what seems to be a remarkable coincidence between the arrival ofprehistoric peoples in North and South America and the time during which mammoths,(10) giant ground sloths, the giant bison, and numerous other large mammals became extinct.Perhaps the human species was driving others to extinction long before the dawn of history.Hunter-gatherers may have contributed to Pleistocene extinctions in more indirectways. Besides over-hunting, at least three other kinds of effects have been suggested:direct competition, imbalances between competing species of game animals, and early(15) agricultural practices. Direct competition may have brought about the demise of largecarnivores such as the saber-toothed cats. These animals simply may have been unableto compete with the increasingly sophisticated hunting skills of Pleistocene people.Human hunters could have caused imbalances among game animals, leading to theextinctions of species less able to compete. When other predators such as the gray wolf(20) prey upon large mammals, they generally take high proportions of each year s crop ofyoung. Some human hunters, in contrast, tend to take the various age-groups of large animalsin proportion to their actual occurrence. If such hunters first competed with the largerpredators and then replaced them. they may have allowed more young to survive each year,gradually increasing the populations of favored species As these populations expanded,(25) they in turn may have competed with other game species for the same environmental niche,forcing the less hunted species into extinction. This theory, suggests that human huntersplayed an indirect role in Pleistocene extinctions by hunting one species more than another. What does the passage mainly discuss?A．The effects of human activities on prehistoric wildlifeB．The origins of the hunter-gatherer way of lifeC．The diets of large animals of the Pleistocene epochD．The change in climate at the end of the Pleistocene epoch

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环境友好型社会，是人与自然和谐发展的社会，应建立的社会体系是 点击查看答案