What happens to the Doppler effect in air (i.e., the shift in frequency of a sound wave)

Question:

What happens to the Doppler effect in air (i.e., the shift in frequency of a sound wave) as the temperature increases?
(a) It is greater at higher temperatures, but only in the case of a moving source and a stationary observer.
(b) It is greater at higher temperatures, but only in the case of a moving observer and a stationary source.
(c) It is greater at higher temperatures than at lower temperatures.
(d) It is less at higher temperatures than at lower temperatures.
(e) The Doppler effect does not change as the temperature increases.