填空题

The difference between a liquid and a gas is obvious under the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fills it to the 41 of a free surface. A gas forms no free surface but 42 to diffuse throughout the space available; it must therefore be kept in a closed container, as in the case of a planet"s atmosphere. The distinction was a 43 feature of early theories describing the phases of matter. In the nineteenth century, for example, one theory 44 that a liquid could be "dissolved" in a vapor without losing its identity, and another theory held that the two phases are made up of different molecules (分子). The theories now 45 take a quite different approach by emphasizing what liquids and gases have in 46 . They are both forms of matter that have no 47 structure, and they both flow easily. They are fluids.
The fundamental similarity of liquids and gases becomes 48 apparent when the temperature and pressure are raised somewhat. Suppose a closed container 49 filled with a liquid is heated. The liquid expands or in other words, becomes less 50 ; some of it evaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and pressure at which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be distinguished.
A. common I. specifically
B. maintained J. prevailing
C. encounter K. dense
D. tends L. prominent
E. partially M. regulation
F. level N. frustrated
G. attempting O. permanent
H. clearly