This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Definitions & Concepts, Subject name: Social & Preventive Medicine
A contraceptive "X" is used by 100 couples for continuous period of 2 years. During this period 20 women become pregnant despite using contraceptive "X". What is the pearl index of contraceptive "X"? 
Select and explain. 
A. 0.1 per HWY, B. 1 per HWY, C. 10 per HWY, D. 1000 per HWY
C: 10 per HWY
Answer: C: 10 per HWY 
Explanation: - The Pearl index is defined as the number of "failures per 100 woman-years of exposure (HWY)." - This rate is given by the formula : - PI = Total accidental pregnancies x 1200/ Total months of exposure In given question: Total accidental pregnancies=20 Total months of exposure=100 couples x 24 months each=2400 months PI = 20 x1200/ 2400 =10 per HWY