Calcium ions and platelets are essential factors for clotting of blood. During some injury, the platelets release inactivated platelet factors that are involved in the activation of other factors involved in the formation of clots in the blood. Calcium ions in the blood catalyze the activation of many factors involved in clot formation like factor VIII, factor IX, factor V and factor XIII. Thus, the correct answer is 'Calcium ions and platelet factors.'

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