2. 外语
  3. The difference between a liquid and a gas is obvious【C1】______the conditions of temperature and pressure commonly found【C2】_____

The difference between a liquid and a gas is obvious【C1】______the conditions of temperature and pressure commonly found【C2】_____

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问题     The difference between a liquid and a gas is obvious【C1】______the conditions of temperature and pressure commonly found【C2】______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【C7】______feature of early theories【C8】______the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be "dissolved" in a vapor without losing its identity, and another theory held that the two phases are【C9】______different kinds of molecules(分子). The theories now prevailing【C10】______a quite different approach by emphasizing【C11】______liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both【C12】______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 as the evaporated molecules are【C17】______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.
【C2】
选项 A、on the surface
B、at the surface
C、on surface
D、at surface
答案B
解析 考查介词短语的含义和用法。本题较具迷惑性的是选项A，但它表示的是“表面上”。只有选项B可接介词 of,组成at the surface of...的结构，表示“在……的表面”，为正确答案。
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大学英语四级

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