A load resistor $R_L$ is connected to a battery of voltage $E$ with internal resistance $R_i$ through a resistance $R_s$ as shown in the figure. For fixed values of $R_L$ and $R_i$, the value of $R_s(\underline{>}0)$ for maximum power transfer to $R_L$ is
- $0$
- $R_L-R_i$
- $R_L$
- $R_L+R_i$