All of the following are features of capillaries that promote exchange between blood and interstitial fluid EXCEPT which one?

A) Capillaries lack connective tissue in their walls.
B) Capillary walls consist of a single layer of squamous epithelial cells.
C) Capillary walls have pores within the cells and slits between the cells, making them "leaky."
D) Capillary walls lack smooth muscle.
E) Capillaries have tiny valves that keep the blood flowing in one direction.