Arteriole
In Lab 2, I told you that structure A is an arteriole. You now know that because there are only one or two layers of smooth muscle in the tunica media. Although one cannot identify veins by the layers of muscle in the wall, you can be confident that a small vein will generally accompany a small artery, and a venule will accompany an arteriole, and so on (although with the muscular and elastic arteries, they are called medium and large veins, respectively).