Thyristor $T_{1}$ is triggered at an angle $\alpha$ (in degree), and  $T_{2}$ at an angle $180^{\circ}+\alpha$, in each cycle of the sinusoidal input voltage. Assume both thyristors to be ideal. To control the load power over the range $0$ to $2\:kW,$ the minimum range of variation in $\alpha$ is:
1. $0^{\circ}$ to $60^{\circ}$
2. $0^{\circ}$ to $120^{\circ}$
3. $60^{\circ}$ to $120^{\circ}$
4. $60^{\circ}$ to $180^{\circ}$