19. 下圖電路 由 a-b 兩端點往左視入之戴維寧等效電壓 VTH及戴維寧等效電阻 RTH各為何？
(A) VTH = 5 V、RTH = 10 Ω
(B) VTH = 5 V、RTH = 20 Ω
(C) VTH = 10 V、RTH = 10 Ω
(D) VTH = 10 V、RTH = 20 Ω。