A single-phase triac based $\text{AC}$ voltage controller feeds a series $\text{RL}$ load. The input $\text{AC}$ supply is $230 \mathrm{~V}, 50 \mathrm{~Hz}$. The values of $\mathrm{R}$ and $\mathrm{L}$ are $10 \Omega$ and $18.37 \mathrm{mH}$, respectively. The minimum triggering angle of the triac to obtain controllable output voltage is
- $15^{\circ}$
- $30^{\circ}$
- $45^{\circ}$
- $60^{\circ}$