30. 如圖( 六 )所示之電路，a、b 兩端的戴維寧等效電壓 Vth ( Thevenin ’s equivalent voltage ) 及等效電阻Rth (Thevenin ’s equivalent resistance )分別為何？ (A) Vth = 16V， Rth = 19 Ω (B) Vth = 24V， Rth = 4 Ω (C) Vth = 8V， Rth = 5 Ω (D) Vth = 16V， Rth = 5 Ω