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: Physiology
[{'content': 'Ions participating in clotting mechanism are: \nSelect and explain. \nA. Iron, B. Copper, C. Calcium, D. Aluminium', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]