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: Psychiatry
IQ is - 
Select and explain. 
A. Mental age / chronological age x 100, B. Chronological age / mental age x 100, C. Mental age + chronological age x 100, D. Mental age - chronological age x 100
A: Mental age / chronological age x 100
Answer: A: Mental age / chronological age x 100 
Explanation: None