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: Social & Preventive Medicine
'Lead time' refers to the time between - 
Select and explain. 
A. Disease onset & final critical diagnosis, B. Disease onset & first possible point of detection, C. First possible point of detection & final critical diagnosis, D. First possible point of detection & usual time of diagnosis
D: First possible point of detection & usual time of diagnosis
Answer: D: First possible point of detection & usual time of diagnosis 
Explanation: Ans. is 'd' i.e., First possible point of detection & usual time of diagnosis