Final answer:
The scale control can be modified to demonstrate lower velocities missing from the Doppler waveform. The wall filter, depth, and gain controls are not directly related to demonstrating missing velocities.
Step-by-step explanation:
When using spectral Doppler, the control that can be modified to demonstrate lower velocities missing from the waveform is the scale. The scale determines the range of velocities that are displayed on the waveform. By adjusting the scale, the sonographer can make sure that lower velocities are visible. For example, if the scale is set too high, it may cut off the lower velocities, but by reducing the scale, those velocities can be seen.
The other controls mentioned in the options are not directly related to demonstrating missing velocities. The wall filter is used to filter out high-frequency noise and artifacts. The depth control is used to adjust the imaging depth. And the gain control is used to adjust the strength of the ultrasound signal.