已知R1:R2:R3=1:9:18,R2与R3并联的电阻为:R23= R2R3 R2+R3 = 9×18 9+18 R1=6R1R2与R3并联后与电阻R1串联,串联电路电压表等于电阻比,故:U1:U2=R1:6R1=1:6故选:A.