A horn produces sound with frequency 0. Let the frequency you hear when you are at rest

A horn produces sound with frequency ƒ0. Let the frequency you hear when you are at rest and the horn moves toward you with a speed u be ƒ1; let the frequency you hear when the horn is at rest and you move toward it with a speed u be ƒ2.

(a) Is ƒ1 greater than, less than, or equal to ƒ2?

(b) Choose the best explanation from among the following:

I. A moving observer encounters wave crests more often than a stationary observer, leading to a higher frequency.

II. The relative speeds are the same in either case. Therefore, the frequencies will be the same as well.

III. A moving source causes the wave crests to "bunch up," leading to a higher frequency than for a moving observer.