A dynamometer arm makes contact with the piezoelectric load cell as shown. The $g-$constant of the piezoelectric material is $50\times10^{-3}\;\text {Vm/N}$ and the surface area of the load cell is $4\;{cm^2}$. If a torque $\tau=20 \;\text{Nm}$ is applied to the dynamometer, the output voltage $\text{V}_\circ$ of the load cell is
- $\text{4 V}$
- $\text{5 V}$
- $\text{10 V}$
- $\text{16 V}$