Which statement about the speed of sound in air is correct?
(a) It remains constant provided the pressure of air is kept constant.
(b) It decreases by about 0.6m/s for every 1∘C rise in temperature.
(c) It is higher at 30∘C than at 0∘C.
(d) It doubles when the temperature is raised from 0∘C to 100∘C.