In a single-loop circuit, an ideal 6.0 V battery is connected in series and opposite orientation to a non-ideal 12 V battery that has an internal resistance r. A 4.0 Ω external resistor completes the loop. When the circuit is closed, a current of 1.0 A flows from the positive terminal of the 12 V battery toward the positive terminal of the 6 V battery.
What is the value of the internal resistance r of the 12 V battery?
a) 4Ω
b) 6Ω
c) 2Ω
d) 8Ω