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
• E.