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 best bio-compatible dental casting alloy is: 
Select and explain. 
A. Co-Cr, B. Ni-Cr, C. Titanium alloy, D. Gold
C: Titanium alloy
Answer: C: Titanium alloy 
Explanation: TITANIUM AND TITANIUM ALLOYS
In vitro evaluation of titanium biocompatibility, percentage attachment efficiency, and proliferation of human fetal fibroblasts and human gingival fibroblasts reveals that a surface layer of titanium oxide (Ti 2 O 3 ) has the ability to coexist with living tissues and organisms. Based on these studies one can conclude that titanium is relatively nontoxic, noninjurious, and not physiologically reactive. Titanium has a light weight, high  strength, and excellent durability when exposed to chemical agents. However, it is susceptible to attack by acidic fluoride products.
Phillips 12 ed page 126