L-Dopa's Efficacy in the Treatment of Parkinson's Disease Decreases After

L-dopa's efficacy in the treatment of Parkinson's disease decreases after a few years of treatment because

A) as the patient gets older plasma concentrations may increase too fast and produce motor disturbances and adverse psychic effects
B) as the patient gets older plasma concentrations may fall too quickly, allowing the disease symptoms to suddenly appear
C) of continued progression of the underlying cause of the disease
D) all of the above