This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: X-RAYS, Subject name: Radiology
[{'content': 'Which of the following is not safe in pregnancy? \nSelect and explain. \nA. USG, B. Doppler, C. MRI, D. CT', 'role': 'user'}, {'content': 'Answer: D \nExplanation: CT uses X-rays so it is not safe during pregnancy. CT scan while pregnant first trimester has the highest risk to the developing foetus due to the carcinogenic and teratogenic effects of the ionizing radiation.', 'role': 'assistant'}]