A 73-year-old man with Parkinson disease presents with tremor, bradykinesia, and rigidity. Vital signs: BP 138/82, HR 88, RR 16, Temp 37.1°C, SpO2 98%. He was started on carbidopa-levodopa three months ago. Current examination shows significant improvement in slowness and rigidity; however, mild resting tremor persists. Levodopa level is therapeutic at 1.2 mcg/mL. Which of the following best explains carbidopa's role in optimizing this medication regimen?

1. A)It directly stimulates dopamine receptors in the striatum
2. B)It inhibits peripheral dopa decarboxylaseGABARITO
3. C)It blocks catechol-O-methyltransferase in the CNS
4. D)It prevents levodopa from crossing the blood-brain barrier
5. E)It inhibits MAO-B in the substantia nigra