A cell with electromotive force (e.m.f.) $E$ and internal resistance $r$ is connected across a resistor of resistance $R$. The resistor is able to dissipate a maximum power $P$ without overheating. What is the greatest value of $E$ for which the resistor does not overheat?
- A$R\sqrt{\frac{P}{(R-r)}}$
- B$R\sqrt{\frac{P}{(R+r)}}$
- C$(R-r)\sqrt{\frac{P}{R}}$
- D$(R+r)\sqrt{\frac{P}{R}}$