【单选题】
3-4求题图3-7所示电路的戴维宁等效电路。<img src=https://huaweicloudobs.ahjxjy.cn/67875014C473A837124B95F7A185DB41.png title=3-4.PNG alt=3-4.PNG/>
①
<img class=jc-formula data-tex={ U }_{ oc }=\frac { 70 }{ 11 } V,\quad { R }_{ eq }=19.55\Omega src=https://huaweicloudobs.ahjxjy.cn/5546F2D093FC14D3AC541417E7A82B33.png style=vertical-align: middle;/>
②
<img class=jc-formula data-tex={ U }_{ oc }=-\frac { 40 }{ 11 } V,\quad { R }_{ eq }=19.55\Omega src=https://huaweicloudobs.ahjxjy.cn/D08F3BF612B181EFF7492CB4CF080C61.png style=vertical-align: middle;/>
③
<img class=jc-formula data-tex={ U }_{ oc }=\frac { 40 }{ 11 } V,\quad { R }_{ eq }=\frac { 50 }{ 11 } \Omega src=https://huaweicloudobs.ahjxjy.cn/23F900BE26E277B31A3AE359A3108B1B.png style=vertical-align: middle;/>
④
<img class=jc-formula data-tex={ U }_{ oc }=-\frac { 70 }{ 11 } V,\quad { R }_{ eq }=19.55\Omega src=https://huaweicloudobs.ahjxjy.cn/4E893A5A3FC46CE66EE93085D7486D0C.png style=vertical-align: middle;/>