8. 如【圖7】所示,其中圖(b)為圖(a)之等效電路,則 Eth 及 Rth 分別為何?(A) Eth =120V, Rth =12Ω(B) Eth = 90V, Rth =12Ω(C) Eth = 20V, Rth = 2Ω(D) Eth =10V, Rth = 2Ω