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 potassium, B. lacks Al, C. lacks Mg, D. lacks Fe
A: Lacks potassium
Answer: A: Lacks potassium 
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 amounts of Fe, Al and Mg than grey MTA.
ref. MTA review, JOE 2010, pg 17