This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
The speed with which the electrons travel from the
filament of cathode to the anode depends upon 
Select and explain. 
A. Potential difference between the two electrodes, B. Number of milliamperes in the tube circuit, C. Angle between the filament and target, D. Voltage of the filament
A: Potential difference between the two electrodes
Answer: A: Potential difference between the two electrodes 
Explanation: The primary functions of the power supply of an X-ray machine are to:

Provide a low-voltage current to heat the X-ray tube filament.
Generate a high potential difference to accelerate electrons from the cathode to the focal spot on the anode. The X-ray tube and two transformers lie within an electrically grounded metal housing called the head of the X-ray machine.

White and Pharoah, pg-6