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: Pediatrics
What is the average gain in lenght in 4 years? 
Select and explain. 
A. 25 cm, B. 50 cm, C. 75 cm, D. 100 cm
B: 50 cm
Answer: B: 50 cm 
Explanation: Expected height = Age (yr) × 6 + 77