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: Dental
[{'content': 'The primary difference between MTA and Portland cement is that, MTA: \nSelect and explain. \nA. Lacks K, B. Lacks Al, C. Lacks Mg, D. Lacks Fe', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The primary difference between both types of MTA and Portland cement are a lack of potassium and presence of bismuth oxide.\xa0\nWhite MTA contains lower amount of Fe, Al and Mg than grey MTA.', 'role': 'assistant'}]