19. 如圖所示,由端點 a、b 向左端電路看入,其戴維寧等效電壓 Vth 與等效電阻 Rth 分別為何?
(A) Vth = 12 V,Rth = 2 Ω(B) Vth = 6 V,Rth = 9 Ω(C) Vth = 12 V,Rth = 9 Ω(D) Vth = 18 V,Rth = 2 Ω