2. 外语
  3. The difference between a liquid and a gas is obvious(1)_____ the conditions of temperature and pressure commonly found(2)_____ o

The difference between a liquid and a gas is obvious(1)_____ the conditions of temperature and pressure commonly found(2)_____ o

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问题     The difference between a liquid and a gas is obvious(1)_____ the conditions of temperature and pressure commonly found(2)_____ of the Earth. A liquid can be kept in an open container and (3)_____ it to the level of a free surface. A gas forms no free surface but(4)_____ to diffuse throughout the space available; it must(5)_____ be kept in a closed container, as(6)_____ a planet’s atmosphere. The distinction was a(7)_____ feature of early theories(8)_____ 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(9)_____ different kinds of molecules. The theories now prevailing(10)_____ a quite different approach by emphasizing(11)_____ liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both(12)_____ easily. They are fluids.
    The(13)_____ similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat.(14)_____ a closed container partially filled with a liquid is heated. The liquid expands, or(15)______, becomes less dense; some of it evaporates.(16)_____, the vapor above the liquid surface becomes denser as the evaporated molecules are(17)_____ it. The combination of temperature and pressure(18)_____ the densities become equal is(19)_____ the critical point. Above the critical point the liquid and the gas can no longer be(20)_____; there is a single, undifferentiated fluid phase of uniform density.
(8)
选项 A、having described
B、described
C、describing
D、to have described
答案C
解析 从句子结构分析，空格处应为后置定语，theory与动词describe之间应该为逻辑上的主动关系，因而只能用现在分词。非谓语动词的完成式一般不用作后置定语，故排除A，选C。
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