The transfer function of a control system is given as $G(s)=\frac{k}{s^2+4s+k}$ where $k$ is the gain of the system in radian$/$amp$.$ For this system to be critically damped, the value of $k$ should be

a$.1$

c$.3$

d$.4$

Roof tocus is a

$($a$.$ Time domain

b$.$ Frequency domain approach

c$.$ Combination of both

d$.$ None of these

$k=h^2$

$$

$$