A power supply is connected to four resistors that have different resistances, as shown. The currents through the four resistors are $I_1$, $I_2$, $I_3$ and $I_4$. Which equation is correct?
- A$I_1 = I_2 - I_3$
- B$I_2 = I_1 + I_4$
- C$I_3 = I_4 - I_1$
- D$I_4 = I_2 + I_3$