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': 'Most accurate assessment of gestational age by USG is done by – \nSelect and explain. \nA. Femur length, B. Gestational sac size, C. Menstrual history, D. Crown rump length', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]