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: Dental
How many days took to form 1mm layer of enamel: 
Select and explain. 
A. 90 days., B. 140 days., C. 240 days., D. 180 days.
C: 240 days.
Answer: C: 240 days. 
Explanation: Mean of daily rate of enamel formation – 3.5µm/day to 4µm/day
So to form 1mm thickness total approximately 240 days needed.