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 'heel effect' results 
Select and explain. 
A. In low intensity X-ray beam on anode side of central ray, B. In high intensity beam of X-rays towards cathode, C. Both of them are true, D. None of the above are true
C: Both of them are true
Answer: C: Both of them are true 
Explanation: None