An RL Series Circuit Has the Following Components: 6 $\Omega$ Resistor, 12-V Battery, Ammeter, and Switch

An RL series circuit has the following components: 6.0-mH coil, 0.50- $\Omega$ resistor, 12-V battery, ammeter, and switch. What is the time constant of this circuit?

A) 2.5 *10^-2 s
B) 12 *10^-3 s
C) 200 s
D) 5.0 *10^-3 s