55 具有 p-(R 4 )C 6 H 4 -CH(R 2 )-CH(R 1 )-NH-R 3 之結構，而呈現最強中樞興奮作用的化合物是：
(A)R 4 =R 3 =R 2 =H, R 1 =CH 3
(B)R 4 =R 2 =OH, R 1 =H, R 3 =CH 3
(C)R 4 =H, R 2 =OH, R 1 =R 3 =CH 3
(D)R 4 =R 2 =OH, R 1 =CH 3 , R 3 =H