A liquid surge tank has $F_{\text {in }}$ and $F_{\text {out }}$ as the inlet and outlet flow rates respectively, as shown in the figure below. $F_{\text {out }}$ is proportional to the square root of the liquid level $h$. The cross-sectional area of the tank is $20 \mathrm{~cm}^2$. Density of the liquid is constant everywhere in the system. At steady state, $F_{\text {in }}=F_{\text {out }}=10 \mathrm{~cm}^3 \mathrm{~s}^{-1}$ and $h=16 \mathrm{~cm}$. The variation of $h$ with $F_{\text {in }}$ is approximated as a first order transfer function. Which one of the following is the $\text{CORRECT}$ value of the time constant (in seconds) of this system?
- $20$
- $32$
- $64$
- $128$