The thicker and more muscular walls of arteries help them to withstand and absorb the pressure waves which begin in the heart and are transmitted by the blood.The larger arteries closer to the heart, receive a large amount of blood and have to be elastic. This is achieved by increased concentration of elastin fibers in tunica media which is the middle layer of the arterial wall. Artery carries the blood from the heart which is pure and it has to travel faster so it exerts more pressure on the wall. this is the reason why artery walls are strong and elastic.
The answer is rr because if it were "Rr" it would be round. Dominant always exceeds the recessive. so if both traits were recessive, it would be recessive. so "rr" is your answer
