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.
【C8】
选项 A、having described
B、described
C、describing
D、to have described
答案C
解析 考查句子层面非谓语动词的使用。从该题的句法结构上看，选择项为一个修饰限定成分，而且theory与动词 describe之间应该为逻辑上的主动关系，因此不能用表被动关系的-ed分词，而只能用-ing分词。又因为句中没有相关的语句强调动作完成的时间或过程，所以不能用非谓语动词的完成式。综上所述，选项C为正确答案，用现在分词的现在时态对某一事实进行“说明”。
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大学英语四级

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