In a Series Circuit Containing a Resistor and an Inductor

Multiple Choice

In a series circuit containing a resistor and an inductor connected to an ideal dc source and a switch, the inductor gets 40% of its maximum current 1.8 s after the switch is closed. What is the
Time constant of this circuit?

A) 2.5 s
B) 0.80 s
C) 3.5 s
D) 5.5 s
E) 1.5 s