The diagram represents a circuit containing a cell and three resistors with resistances $R_1$, $R_2$ and $R_3$. The cell has negligible internal resistance. The total resistance of the circuit is $R_T$. Which equation for $R_T$ is correct?
- A$R_T = R_1 + R_2 + R_3$
- B$R_T = \frac{1}{R_1 + R_2} + \frac{1}{R_3}$
- C$\frac{1}{R_T} = \frac{1}{R_1 + R_2 + R_3}$
- D$\frac{1}{R_T} = \frac{1}{R_1 + R_2} + \frac{1}{R_3}$