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
[{'content': 'Contraceptive efficacy expressed as - \nSelect and explain. \nA. 100 women months, B. 1000 women years, C. 100 women years, D. 10 women years', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]