A 42-year-old patient is scheduled for surgery that will likely require a transfusion. Because the patient has a rare blood type, an autologous blood transfusion is planned. Prior to surgery, 1500 mL of blood is collected. The collection tubes contain calcium citrate to prevent coagulation. Which of the following is the mechanism for citrate’s anticoagulative action? A: Activating plasminogen B: Binding factor XII C: Blocking thrombin D: Chelating calcium