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': 'Parameters used to estimate gestational age in last trimester – a) CR lengthb) Abdominal circumferencec) BPDd) Femur length \nSelect and explain. \nA. abc, B. acd, C. ac, D. bcd', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Estimation of age by USG\n\n1st trimester :- Crown - rump length (CRL).\n2nd trimester :- Femoral length (FL), Biparietal diameter (BPD)\n3rd trimester :- Femoral length, Biparietal diameter, abdominal circumference (AC), head circumference (HC).', 'role': 'assistant'}]