Why Do the Arteries and Veins of the Pulmonary Circuit

Why do the arteries and veins of the pulmonary circuit have thinner walls than arteries and veins of the systemic circuit?

A) The pressure against the walls of the vessels is much lower in the pulmonary circuit than the systemic circuit.
B) The pulmonary arteries lack a tunica externa.
C) The systemic arteries have fewer elastic fibers than the pulmonary arteries.
D) The tunica media is significantly thinner in the systemic circuit than the pulmonary circuit.