3 圖示為理想運算放大器電路,若 R1 = 1 kΩ、R2 = 3 kΩ、Ra = 1 kΩ、Rb = 3 kΩ,v1 = 4 V,v2 = -2 V,則輸出電壓 vO為若干 V?(A) 0(B) 5(C) 10(D) 12