[单项选择]

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 fill it to the level of a free surface. A gas forms no free surface but tends to diffuse(扩散)throughout the space available; it must therefore be kept in a closed container or held by a gravitational field, as in the case of a planet’s atmosphere. The distinction was a prominent feature of early theories describing 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 that held the two phases are made up of different kinds of molecules. liquids and gases. The theories now prevailing take 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 readil
A. is affected by changes in pressure
B. has a permanent structure
C. forms a free surface
D. is considerably more common

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 fill it to the level of a free surface. A gas forms no free surface but tends to diffuse(扩散)throughout the space available; it must therefore be kept in a closed container or held by a gravitational field, as in the case of a planet’s atmosphere. The distinction was a prominent feature of early theories describing 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 that held the two phases are made up of different kinds of molecules. liquids and gases. The theories now prevailing take 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 readil
A. The liquid and the gas phases become more similar.
B. The liquid and the gas become less dense.
C. The container expands.
D. The liquid evaporates out of the container.