An e.m.f. source $E$ has a fixed internal resistance $r$ and is connected to an external variable resistor of resistance $R$. As $R$ is raised from a value smaller than $r$ to a value larger than $r$, which statement is correct?
- AThe terminal potential difference remains constant.
- BThe current in the circuit increases.
- CThe e.m.f. of the source increases.
- DThe largest output power is obtained when $R$ reaches $r$.