On a summer afternoon, the time gap between a shout and its echo from a hilltop is observed to be smaller than on a cold winter morning. Which of the following best explains this observation?
(a) Sound of greater amplitude (louder) travels faster through air.
(b) Due to thermal expansion, the hill is slightly closer on a hot afternoon.
(c) Sound travels faster at higher air temperatures.
(d) Higher temperature raises the frequency of the sound, and waves of higher frequency move faster.