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': 'The influence of maternal smoking and LBW incidence is studied. Detailed smoking history is taken at first antenatal visit and smoking history and birth weight were studied later. The type of study is \nSelect and explain. \nA. Retrospective cohort study, B. Cross sectional study, C. Clinical trial, D. Prospective cohort study', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]