2. 外语
  3. 　　 The difference between a liquid and a gas is obvious【B1】______ the conditions of temperature and pressure commonly found at

　　 The difference between a liquid and a gas is obvious【B1】______ the conditions of temperature and pressure commonly found at

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问题 　　  The difference between a liquid and a gas is obvious【B1】______ the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and 【B2】______ it to the level of a free surface. A gas forms no free surface but 【B3】______ to diffuse throughout the space available; it must【B4】______ be kept in a closed container, as【B5】______ a planet’s atmosphere. The distinction was a prominent feature of early theories 【B6】______ 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    【B7】______  different kinds of molecules (分子). The theories now prevailing    【B8】______ a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both flow easily. They are fluids.
   The 【B9】______ similarity of  liquids  and  gases  becomes  clearly apparent  when the temperature and pressure are raised somewhat.【B10】______  a closed container partially filled with a liquid is heated.  The liquid expands,  or 【B11】______ ,  becomes less dense;  some of it evaporates.【B12】______ , the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and pressure【B13】______ the densities become equal is 【B14】______ the critical point. Above the critical point the liquid and the gas can no longer be 【B15】______  ; there is a single, undifferentiated fluid phase of uniform density.
【B2】
选项 A、fills
B、be filled
C、filling
D、to fill
答案A
解析 语法知识。由于所填词前有 and 这一并列连词，因此可以排除C与D，因为二者与前面的成分无法构成并列结构。由于所填词后有 it 这一宾语，故可排除B，因为被动式后不会再出现宾语，除非 fill 为带双宾语的动词，如果将 fill 看作那样的动词，由于 it指 container，显然这会使句子在意思上讲不通。这里，作者使用一般现在时(fills)，表示一般属性。注意此处 fill 不涉及人为因素，与"We can fill the liquid to the level of a freesurface"
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