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
Density of a radiograph depends on : 
Select and explain. 
A. Thickness of  object, B. The quality and quantity of X-rays., C. The velocity of e - emitted from the cathode., D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Density / Overall darkening of a radiograph depends on:

Thickness of object
The quality and quantity of X-rays.
The velocity of e - emitted from the cathode.