Mark the pair of substances among the following which is essential for coagulation of blood.

Answer: CSolution: Certain factor released by the tissues at the site of injury initiate coagulation. Calcium ions play a very important role in clotting. Blood clotting can also be prevented in a test tube by adding a little oxalate or citrate. Oxalate or citrate reacts with calcium to form an insoluble compound, so free calcium ion necessary for clotting are not mentioned in the option (c)