21 下列反應稱為 Knorr 吡咯合成,產物經由脫水縮合過程而得,結構式中的 R
1~R
4 為何?

(A) R1 = CH3 ; R2 = CO2C2H5 ; R3 = CH3 ; R4 = CO2C2H5
(B) R1 = CH3 ; R2 = CO2C2H5 ; R3 = CO2C2H5 ; R4 = CH3
(C) R1 = CO2C2H5 ; R2 = CH3 ; R3 = CH3 ; R4 = CO2C2H5
(D) R1 = CH3 ; R2 = CH3 ; R3 = CO2C2H5 ; R4 = CO2C2H5