Carbidopa, a dopa-decarboxylase inhibitor, is often used in the treatment of parkinsonism because it

Decreases the peripheral metabolism of levodopa

Potentiates the central action of norepinephrine

Potentiates the central action of dopamine

Inhibits the peripheral stimulatory fibers from the centralner vous system

Increases the permeability of the blood-brain barrier to levodopa