26. Blood fails to clot while flowing in the blood vessel because of the presence of :

(A) Heparin

(B) Prothrombin

(C) Haemoglobin 

(D) Hirudin

Ans. (A) Heparin

Heparin is an anticoagulant (blood thinner) that prevents the formation of blood clots. Heparin is used to treat and prevent blood clots in the veins, arteries, or lung. Heparin is also used before surgery to reduce the risk of blood clots.