[单选题]
A traveling wave is generated on a string tied to a speaker as shown.
Which of the following statements are correct about the travelling wave?
I. If the frequency of the sound decreases, the wavelength of the travelling wave increases.
II. If the speaker emits a louder sound the amplitude of the travelling wave increases.
III. If the frequency of the sound increases, the travelling wave moves along the string faster.
A. Only $I$
B. $I$ and $II$ only
C. $II$ and $III$ only
D. $I$, $II$, and $III$
