Correct Response: D. Refraction occurs when one part of a wave front travels at a greater speed than another part of a wave front. For example, if the temperature is higher near the ground, the part of the sound wave front near the ground will travel faster, causing the sound wave to refract upward. Rainfall (A) and moving air (B) would not cause refraction of a sound wave unless there were differences across the wave front in the amount of rainfall or moving air. Regions of sun and shade (C) would only cause refraction indirectly, and only if they resulted in temperature differences.
