In El 02.04, which of the follow is the time constant? \text { a. } \frac{R_{1}}{L}+\frac{R_{2}}{L} \text { b. } \frac{L}{R_{1}}+\frac{L}{R_{2}} \text { c. } \frac{1}{R_{1}}+\frac{1}{R_{2}} \text { d. }

\frac{L R_{2}}{R_{1}}+\frac{L R_{1}}{R_{2}}