C11: Three observers, A, B, and C are listening to a moving source of sound. The diagram below shows the location of the wavecrests of the moving source with respect to the three observers. Which of the following is true?

1. The wavefronts move faster at A than at B and C.

2. The wavefronts move faster at C than at A and B.

3. The frequency of the sound is highest at A.

4. The frequency of the sound is highest at B.

5. The frequency of the sound is highest at C.