The open loop transfer function of a unity negative feedback system is $G(s)=\frac{k}{s\left(1+s T_1\right)\left(1+s T_2\right)}$, where $k, T_1$ and $T_2$ are positive constants. The phase cross-over frequency, in $\text{rad/s,}$ is
- $\frac{1}{\sqrt{T_1 T_2}}$
- $\frac{1}{T_1 T_2}$
- $\frac{1}{T_1 \sqrt{T_2}}$
- $\frac{1}{T_2 \sqrt{T_1}}$