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.
【C7】
选项 A、having described
B、described
C、describing
D、to have described
答案C
解析 动词的不同形式。在这里describe作为修饰theories的后置定语，意思是描述物质状态的理论，所以用describing。
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