A wave disturbance travelling in air enters a medium in which its velocity is less than that in air. Which of the following statements is true about the wave medium?
A. both the frequency of the wave and the wavelength are decreased B. the frequency of the wave is decreased while the wavelength is increased C. the frequency of the wave is unaltered while the wavelength is decreased D. the frequency of the wave is decreased while the wavelength is unaltered
