Which of the following statements is false?

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(D) $1$. The wavelength of sound is defined as $\lambda = v/f$. Since the source determines the frequency $f$ and the medium determines the speed $v$,

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When only the medium moves from the source to the observer,the frequency heard by the observer will be greater than the original frequency.

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(D) $1$. The wavelength of sound is defined as $\lambda = v/f$. Since the source determines the frequency $f$ and the medium determines the speed $v$,the wavelength is independent of the observer's motion. Thus,statement $A$ is true.$2$. When the source moves,the distance between successive wave crests changes,which directly affects the wavelength. Thus,statement $B$ is true.$3$. If the medium moves with velocity $u$,the speed of sound relative to the ground becomes $v' = v \pm u$. The wavelength also changes to $\lambda' = (v \pm u)/f$. Since $f$ is constant,the ratio $\lambda'/v' = 1/f$ remains constant,meaning they change in equal proportion. Thus,statement $C$ is true.$4$. When the medium moves from source to observer,the effective speed of sound increases to $v' = v + u$. The frequency heard by the observer is $f' = v'/\lambda$. Since $\lambda = v/f$,we get $f' = (v+u)/(v/f) = f(1 + u/v)$. This is greater than $f$. However,in the Doppler effect,if the source and observer are at rest relative to the ground,the frequency heard is $f' = (v+u)/((\lambda)) = (v+u)/(v/f) = f(1+u/v)$. Wait,the frequency heard by a stationary observer from a stationary source when the medium moves is actually the same as the original frequency because the number of waves passing per second remains constant. Therefore,statement $D$ is false.

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