$4.($a$)$ Table C$1$ shows three situations in which the Doppler effect may arise. The first two columns give the velocity vector of the sound source and the observer, where the length of each arrow is proportional to the speed.

For the given situation $($I$),($II$)$ and $($III$),$ fill in the empty columns by deciding whether the wavelength of the sound and the speed of the sound as perceived by the observer increase, decrease or remain the same compared to the case when there is no Doppler effect.

Table C$1$