The diagram illustrates a circuit that has four voltmeters. Their readings are $V_1$, $V_2$, $V_3$ and $V_4$. Every reading is positive. Which equation linking the voltmeter readings is correct?
- A$V_1 = V_2 + V_4$
- B$V_1 = V_2 + V_3 + V_4$
- C$V_2 + V_3 = V_4$
- D$V_3 + V_4 - V_2 = 0$