该电路的回路电流方程,正确的是?
A、(R1+R2)Ia -R2Ib = -US1+ US2;
-R2Ia + (R2+R3)Ib - R3Ic = -US2;
-R3Ib + (R3+R4)Ic = US4;
B、(R1+R2)Ia -R2Ib = US1- US2;
-R2Ia + (R2+R3)Ib - R3Ic = US2;
-R3Ib + (R3+R4)Ic = -US4;
C、(R1+R2)Ia +R2Ib = US1- US2;
R2Ia + (R2+R3)Ib - R3Ic = US2;
R3Ib + (R3+R4)Ic = -US4;
D、R1Ia -R2Ib = US1- US2;
-R2Ia + (R2+R3)Ib - R3Ic = US2;
-R3Ib + R3Ic = -US4;