15. 如圖(七)所示之JFET電晶體電路,已知該電晶體截止電壓VGS(off) = – 5V,直流閘源極電壓VGS = –4V時,ID =0.5mA,則R1 /R2值為何?(A) 0.5(B) 1(C) 2(D) 4