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
Which one of the following elements is NOT added in small amount to be used as grain refiners? 
Select and explain. 
A. Iridium, B. Ruthinium, C. Indium, D. Platinum
D: Platinum
Answer: D: Platinum 
Explanation: None