Why do veins have relatively thin walls? A)It allows a typical vein to change the diameter of its lumen. B)It allows a typical vein to be stretched as its diameter increases during ventricular systole. C)A typical vein's wall allows exchange to occur quickly by diffusion across endothelial cells or through gaps between adjacent endothelial cells. D)A typical vein does not need to withstand much pressure. E)There are sphincters associated with the walls of veins, which allow vasomotion to occur.