2014年4月托福考试模拟试卷及答案(第四套)_第20页

　　This vertical movement of the fieldstones is not simply an artifact of soil erosion; it is the result of frost heaving. In the fall the soil freezes first beneath stones, because stones are a better conductor of heat than soil. Or, put another way, soil is a better insulator than rock. In a sea of insulation, stones are chilly islands.

　　Because most glacial till has a fairly high water content, ice forms beneath fieldstones when they freeze, and the expansion of this ice forces them upward. Even when the ice thaws, the stones do not return to their original positions because during thawing particles of soil seep into the cavity beneath， partially preventing the stones from dropping. Like a ratchet on a car jack each freeze-thaw cycle gradually lifts the fieldstones toward the surface. In a very cold winter there may actually be two thrusts per freeze. Ice expands when it initially forms, but as the temperature plummets, the ice contracts. In the reverse process, when this very cold ice finally melts, it must expand a second time, pushing the stone once more. In theory, the upward movement of fieldstones should result in pure soil, all the stones above the frost line having been pushed to the surface and carried away. What a vision! Acres of pure, deep soil and crowbars rusting away unused. Alas, the fastest stones move only an inch or so a year, and most are orders of magnitude slower.

　　What statement was most probably made in the paragraph preceding this passage?

　　(A) Pure soil is quickly eroded.

　　(B) Fieldstones are lifted to the top of the soil.

　　(C) It is not easy to move stones from fields.

　　(D) Ancient cultural artifacts are buried deep in the soil.

　　Passage 23.

　　Just how salt became so crucial to our metabolism is a mystery; one appealing theory trace sour dependence on it to the chemistry of the late Cambrian seas. It was there, a half-billion years ago, that tiny metazoan organisms first evolved systems for sequestering and circulating fluids. The water of the early oceans might thus have become the chemical prototype for the fluids of all animal life-the medium in which cellular operations could continue no matter how the external environment changed. This speculation is based on the fact that, even today, the blood serums of radically divergent species are remarkably similar. Lizards, platypuses, sheep, and humans could hardly be more different in anatomy or eating habits, yet the salt content in the fluid surrounding their blood cells is virtually identical.

　　As early marine species made their way to freshwater and eventually to dry land, sodium remained a key ingredient of their interior, if not their exterior, milieu. The most successful mammalian species would have been those that developed efficient hormonal systems for maintaining the needed sodium concentrations. The human body, for example, uses the hormonesrenin, angiotensin, and aldosterone to retain or release tissue fluids and bloodplasma. The result, under favorable conditions, is a dynamic equilibrium in which neither fluid volume nor sodium concentration fluctuates too dramatically. But if the body is deprived of salt, the effects soon become dangerous, despite compensatory mechanisms.

　　What did the paragraph preceding the passage most probably discuss?

　　(A) Methods of mining salt

　　(B) Ancient beliefs about the powers of salt

　　(C) How humans used salt during the Cambrian period

　　(D) The importance of salt to our metabolism

　　Passage 24.

　　The origins of the horse go back to eohippus, the "dawn-horse" of the Eocene, only 10to 20 inches tall. Like its relatives, the ancient tapir and rhinoceros, eohippus had four toes on its front feet, three on the rear, and teeth adapted to a forest diet of soft leaves. Eohippus died out about 50 million years ago in both North America and Europe.

　　Later ancestral horse types moved from their forest niche out onto the grassy plains. Their teeth had to accommodate to hard siliceous grass. No longer could these proto horses slip away through thick forest when danger threatened. Escape now demanded speed and endurance. Limbs grew longer. Extra toes became vestiges that were not visible externally.

　　The paragraph following the passage most probably discusses

　　(A) other changes that the rhinoceros has undergone

　　(B) more reasons for the extinction of eohippus

　　(C) further development of early horse types

　　(D) the diet of eohippus