30. In class, even if the teacher is lecturing with their back to you, you can still hear the sound, because the direction of sound propagation is towards $\qquad$
A. one direction
B. front and back
C. left and right
D. all directions
【Answer】D
【Analysis】Sound relies on the vibration of media such as air and water for propagation, so it cannot propagate in a vacuum. The direction of propagation is determined by the direction of medium vibration, which can be omnidirectional or "directed." However, due to the longer wavelength of sound waves, they can bend around objects (diffraction phenomenon), which is why you can "hear someone before you see them." When a teacher lectures in a classroom, the sound mainly propagates through the air, and the direction of propagation is omnidirectional. Of course, some sound is also reflected by the classroom walls. Therefore, the correct answer is D.