Two identical batteries, each of emf 6.0 V and internal resistance 0.20 Ω, are connected in series and used to power a single 4.0 Ω resistor. What is the efficiency of this system, defined as the percentage of total power produced by the batteries that is dissipated in the external resistor?
a) 95%
b) 100%
c) 91%
d) 90%