This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Social and Language Milestones, Subject name: Pediatrics
Child knows his/her sex at the age of: 
Select and explain. 
A. 1 year, B. 3 years, C. 5 years, D. 7 years
B: 3 years
Answer: B: 3 years 
Explanation: A child knows her full name and gender by the age of 3 years.