如图示理想运算放大电路中,R<sub>1</sub>=1kΩ,R<sub>F</sub>=3kΩ,U<sub>i</sub>=1V,则U<sub>1</sub>、U<sub>2</sub>、U<sub>3</sub>、U<sub>4</sub>中电压为3V的是()<img src="https://nimg.ppkao.com/2018-09/wangjing/2018090615375920831.jpg?sign=174cadbb5eb01e3017023b49429e6290&t=62d2aa77" />
A、U1
B、U2
C、U3
D、U4