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: Biochemistry
The function of vitamin A is/are related to which of the following? 
Select and explain. 
A. Retinol, B. Retinal, C. Retinoic acids, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None