Part II cioze [15 min.](15% ）
 

　　Decide which of the choices given below would best complete the passage if inserted in the corresponding blanks. Mark the best choice for each blank on your answer sheet.
The difference between a liquid and a gas is obvious (21) the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and (22) it to the level of a free surface. A gas forms no free surface but (23) to diffuse throughout the space available: it must (24) be kept in a closed container, as (25) a planet’s atmosphere. The distinction was a prominent feature of early theories (26) 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 (27) different kinds o f molecules. The theories now prevailing (28) a quit e 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 (29) similarly of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. (30) a closed container partially filled with a liquid is heate
D. The li quid expands or (31), becomes less dense; some of it evaporates. (32), the vapor above the liquid surface becomes dense r as the evaporated molecules are added to it. The combination of temperature and pressure (33) the densities become equal is (34) the critical point. Above the critical point the liquid and the gas can no longer be (35); there is a single, undifferentiated fluid phase of uniform density.
21. A. in 
B. on 
C. under 
D. beyond
22. A. fills
B. be filled 
C. filling 
D. to fill
23. A. intends 
B. tends 
C. inclines 
D. contends
24. A. however 
B. nevertheless 
C. so 
D. therefore
25. A. in the event of 
B. in the case of 
C. with a view to 
D. with reference to
26. A. having described 
B. described 
C. describing 
D. to have described
27. A. made up of 
B. consisted of 
C. constituted of 
D. made from
28. A. apply 
B. adapt 
C. take 
D. conduct
29. A. elementary 
B. crucial 
C. rudimentary 
D. fundamental
30. A. Suppose 
B. To suppose 
C. Being supposed 
D. Supposed
31. A. in a word 
B. in the meantime 
C. in other words 
D. in that case
32. A. Similarly 
B. In contrast 
C. Furthermore 
D. Instead
33. A. on that 
B. on which 
C. at that 
D. at which
34. A. known 
B. defined 
C. called 
D. referred to
35. A. classified 
B. recognized 
C. categorized 
D. distinguished