A power of 50.0 W is supplied to a resistor R = 5.0 Ω by a real cell of e.m.f. E1 = 18.0 V. What is the internal resistance of the actual battery?
a. 3.6 Ω
b. 5.0 Ω
c. 7.2 Ω
d. 10.0 Ω