This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
Child knows his/her sex by the age of? 
Select and explain. 
A. 2 years, B. 3 years, C. 4 years, D. 5 years
B: 3 years
Answer: B: 3 years 
Explanation: O.P. Ghai 7th ed, page 30