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
The primary difference between MTA and Portland cement is that, MTA: 
Select and explain. 
A. Lacks K, B. Lacks Al, C. Lacks Mg, D. Lacks Fe
A: Lacks K
Answer: A: Lacks K 
Explanation: The primary difference between both types of MTA and Portland cement are a lack of potassium and presence of bismuth oxide. 
White MTA contains lower amount of Fe, Al and Mg than grey MTA.