An infinitely long cylindrical water filament of radius $R$ is surrounded by air. Assume water and air to be static. The pressure outside the filament is $P_{\text {out }}$ and the pressure inside is $P_{\text {in }}$. If $\gamma$ is the surface tension of the water-air interface, then $P_{\text {in }}-P_{\text {out }}$ is
- $\frac{2 \gamma}{R}$
- $0$
- $\frac{\gamma}{R}$
- $\frac{4 \gamma}{R}$