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: Radiology
[{'content': 'USG done at 18 - 20 weeks mainly to : \nSelect and explain. \nA. Detect fetal abnormality, B. Determine sex, C. Estimate liquor, D. Determine maturity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: At 18 - 20th weeks, 2nd scan is done mainly to look for congenital anomalies.', 'role': 'assistant'}]