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: Physiology
Which in the following when increased can cause a shift of the oxygen-hemoglobin dissociation curve to the left? 
Select and explain. 
A. pH, B. Temperature, C. 2,3-BPG, D. pCO2
A: pH
Answer: A: pH 
Explanation: Ans. a. pH