A Series RL Circuit with Inductance L, Resistance R, and a Switch

Multiple Choice

A series RL circuit with inductance L, resistance R, and a switch is connected to an emf V. The current in the circuit is zero when the switch is closed at $t = 0$ s. The current increases to 70% of its final value in 4.0 s. The time constant of the circuit is

A) 2.8 s.
B) 5.7 s.
C) 8.5s.
D) 11 s.
E) 18 s.