A battery has an e.m.f. $E$ and internal resistance $r$. It supplies a current $I$ to a variable resistor, and the p.d. at its terminals is $V$. The variable resistor is adjusted so that $I$ gets larger. Why does $V$ fall?
- AThe e.m.f. $E$ decreases.
- BThe internal resistance $r$ increases.
- CThe p.d. across $r$ increases.
- DThe resistance of the variable resistor increases.