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: Pathology
[{'content': 'C1 is controlled by? \nSelect and explain. \nA. Calcium, B. Magnesium, C. Manganese, D. Phosphate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The initial enzyme, Cl, is a complex formed through a calcium-dependent association between\xa0two reversibly interacting subunits, Clq and Clr2s2.\nIn the absence of an immune response, approximately 70% of Cl exists as a complex between\xa0these two subunits.\nCl occurs in serum as a proenzyme that tends to undergo autoactivation (in which the two\xa0subunits are bound together) but is strictly controlled by Cl-inhibitor (Cl-In or Cl esterase).', 'role': 'assistant'}]