A whistle used to train dogs produces sound of frequency 30 kHz. Dogs can hear it but a person standing nearby cannot. The most appropriate reason is that
(a) the frequency is above the upper limit of human hearing
(b) the sound intensity is below the threshold of human hearing
(c) the wavelength is too short to be detected by the larger human ear canal
(d) the whistle actually produces infrasonic waves that people cannot perceive