A:R1I1–R2I2+R3I3–R4I4=US1+US2–US3–US4 B:R1I1–R2I2–R3I3+R4I4=US1–US2–US3+US4 C:R1I1–R2I2–R3I3+R4I4=US1+US2–US3–US4 D:–R1I 点我阅读全文