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
A patient who has had a recent amalgam filling in the upper teeth has a gold filling in the lower teeth, the patient complains of pain. The reason for this can be mainly 
Select and explain. 
A. improper amalgam filling, B. pulp exposure, C. galvanism, D. none of the above
C: galvanism
Answer: C: galvanism 
Explanation: None