Which blood vessel tunic has the most active role in controlling blood flow?

• A. Tunica intima, as it is makes direct contact with the blood.
• B. Tunica media, as it contains smooth muscle which causes the vessel’s diameter to change.
• C. Tunica externa, as it dampens the large pressure changes caused by the beating of the heart.
• D. None of the layers play an active role in controlling blood flow.