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
C1 is controlled by? 
Select and explain. 
A. Calcium, B. Magnesium, C. Manganese, D. Phosphate
A: Calcium
Answer: A: Calcium 
Explanation: The initial enzyme, Cl, is a complex formed through a calcium-dependent association between two reversibly interacting subunits, Clq and Clr2s2.
In the absence of an immune response, approximately 70% of Cl exists as a complex between these two subunits.
Cl occurs in serum as a proenzyme that tends to undergo autoactivation (in which the two subunits are bound together) but is strictly controlled by Cl-inhibitor (Cl-In or Cl esterase).