2. 外语
  3. The difference between a liquid and a gas is obvious 【C1】______the conditions of temperature and pressure 【C2】______found at the

The difference between a liquid and a gas is obvious 【C1】______the conditions of temperature and pressure 【C2】______found at the

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问题     The difference between a liquid and a gas is obvious 【C1】______the conditions of temperature and pressure 【C2】______found at the surface of the Earth. A liquid can be kept in an open container and 【C3】______ it to the level of a free surface. A gas forms no free surface but 【C4】______to diffuse throughout the space available; it must 【C5】______be kept in a closed container, as 【C6】______a planet’s atmosphere. The distinction was a prominent feature of early theories 【C7】______the phases of matter. In the nineteenth century, for example, one theory 【C8】______that a liquid could be "dissolved" in a vapor without losing its 【C9】______, and another theory held that the two phases are 【C10】______different kinds of molecules (分子). The theories now prevailing 【C11】______a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no 【C12】______structure, and they both flow easily. They are fluids.
    The 【C13】______similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. 【C14】______a closed container partially filled with a liquid is heated. The liquid expands, or 【C15】______, becomes less dense; some of it evaporates. 【C16】______, the vapor above the liquid surface becomes denser 【C17】______the evaporated molecules are added to it. The combination of temperature and pressure 【C18】______the densities become equal is 【C19】______the critical point. Above the critical point the liquid and the gas can no longer be 【C20】_____; there is a single, undifferentiated fluid phase of uniform density.
【C9】
选项 A、nature
B、identity
C、position
D、form
答案B
解析 词义。nature是“性质、自然状态”的意思，这里水跟水蒸气都是水，就无所谓性质的改变与否，故不选A：而position是“位置、立场、形势”的意思，用在这里完全跟文意不符，也不选：form是“形状、形式、形态”的意思，但水跟水蒸气很明显在形态上是不一样的，所以最后得出正确答案为B。
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