In the circuit diagram, the ammeter gives a zero reading. The four resistors are of different resistance values $R_1$, $R_2$, $R_3$ and $R_4$. Which equation is correct?
- A$R_1 - R_3 = R_2 - R_4$
- B$R_1 \times R_3 = R_2 \times R_4$
- C$R_1 - R_4 = R_2 - R_3$
- D$R_1 \times R_4 = R_2 \times R_3$