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