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: Gynaecology & Obstetrics
Ovulation is presumed to have occurred if progesterone level is __________ on day 21 
Select and explain. 
A. 0.3 ng / mL, B. 3 ng / mL, C. 30 ng / mL, D. 300ng / mL
B: 3 ng / mL
Answer: B: 3 ng / mL 
Explanation: Ovulation can be safely assumed to have occurred if progesterone level exceeds 3 ng / mL on cycle day 21.