Multiple choice In the process of blood clotting the antithrombin is activated by aspirin warfarin heparin P2Y receptor GPIIb/IIIa receptor antagonists Reveal answer Fill a bubble to check yourself C Correct answer Explanation Heparin is a highly effective anticoagulant. Unfractionated heparin activates antithrombin which inactivates thrombin and factors IXa and Xa.