How is the embolism of a thrombotic stroke formed? ① Blood flow slows down. ② A sudden change in heart rhythm prevents the upper chambers of the heart from contracting normally. ③ Platelets, clotting factors, and fibrin stick together. ④ The clot can be carried up towards the arteries and blood vessels supplying the brain until it can’t squeeze through. A: ②④③① B: ②①③④ C: ②①④③ D: ①②④③