Which statement defines e.m.f.?
A. The e.m.f. of a source is the energy transferred when charge is driven through a resistor.
B. The e.m.f. of a source is the energy transferred when charge is driven round a complete circuit.
C. The e.m.f. of a source is the energy transferred when unit charge is driven round a complete circuit.
D. The e.m.f. of a source is the energy transferred when unit charge is driven through a resistor.