After the skin is cut and a capillary wall breaks, what's the first step in working to form a clot?
a. a set of reactions are triggered. b. thrombin causes sticky fibrin filaments to form the clot. c. the clot seals the area to prevent further blood loss. d. platelets clump at the site.

The best answer is A. As soon as the blood vessel wall is broken or damaged, a series of reactions activate platelets so that they stick to the site of injury. The platelets are able to hold to the blood vessel wall by means of a protein substance called von Willebrand's factor. Collagen and thrombin then act at the site of injury and induce platelets to clump or stick together, forming a mesh that plugs the broken part. Special clotting protein help sequester more platelets to the plug which finally becomes a blood clot.