A simple RL circuit is shown in the following diagram.

For all times t < 0, the switch is in a position such that the 40 kV DC source is in series with the inductor and resistor. At t = 0, the switch is thrown to the left so that the voltage source is bypassed and replaced by a short circuit. Which of the following expressions correctly gives the voltage drop across the resistor, v _R (t), for t > 0?