The solution is option 3
Volume inflow rate = volume anflow rate
$
\pi R^{2} V=n \pi r^{2} \quad \Rightarrow \quad v=\frac{\pi R^{2} V}{n \pi r^{2}}=\frac{V R^{2}}{\mathrm{nr}^{2}}
$
The solution is option 3
Volume inflow rate = volume anflow rate
$
\pi R^{2} V=n \pi r^{2} \quad \Rightarrow \quad v=\frac{\pi R^{2} V}{n \pi r^{2}}=\frac{V R^{2}}{\mathrm{nr}^{2}}
$