[单选题]
For an open pipe, the frequency of the first harmonic is $f$ and its wavelength is $λ$.
For a given harmonic of the same pipe, the ranges of motion of the particles in the pipe are shown below.
Three statements about the standing wave shown in this pipe are
I.The frequency is $2f$
II.The wavelength is $\frac{λ}{4}$
III. The wavelength is less than λ
Which statements are true?
A. $I$ and $II$ only
B. $I$ and $III$ only
C. $II$ and $III$ only
D. $I$, $II$, and $III$
