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
Contraceptive efficacy expressed as - 
Select and explain. 
A. 100 women months, B. 1000 women years, C. 100 women years, D. 10 women years
C: 100 women years
Answer: C: 100 women years 
Explanation: None