(a) In the given figure, a steady current I flows through the circuit when points A and C are connected by a wire of negligible resistance. Find the potential difference between points B and C. (Figure shows points A, B, and C with $E_1=6\text{ V}$, $1\text{ }\Omega$ and $E_2=4\text{ V}$, $3\text{ }\Omega$). OR (b) A battery of emf $21\text{ V}$ and internal resistance $3\text{ }\Omega$ is connected to a resistor. If the current in the circuit is $3\text{ A}$, find : (i) the resistance of the resistor. (ii) the terminal voltage of the battery.