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: Gynaecology & Obstetrics
[{'content': 'Study of fetal pas in first trimester with least radiation hazard : \nSelect and explain. \nA. X-ray abdomen, B. Pelvimetry, C. CT scan, D. Ultrasound', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ultrasound', 'role': 'assistant'}]