다음 주어진 문장이 들어갈 위치로 가장 적절한 것은? Rather, waves fill regions of space, and their evolutions in time are not described by simple trajectories. Broadly speaking, a wave is a disturbance that propagates through space. ( A ) Most waves move through a supporting medium, with the disturbance being a physical displacement of the medium. ( B ) The time dependence of the displacement at any single point in space is often an oscillation about some equilibrium position. For example, a sound wave travels through the medium of air, and the disturbance is a small collective displacement of air molecules―individual molecules oscillate back and forth as the wave passes. ( C ) Unlike particles, which have well-defined positions and trajectories, waves are not localized in space. ( D ) Nevertheless, some waves are more localized than others, and so it is useful to distinguish two broad classes.