A small uniform tube is bent into a circle of radius $r$ whose plane is vertical. The equal volumes of two fluids whose densities are $\rho$ and $\sigma(\rho>\sigma)$, fill half the circle. Find the angle that the radius passing through the interface makes with the vertical:
(a) $\cot \theta=\frac{\rho-\sigma}{\rho+\sigma}$
(b) $\tan \theta=\frac{\rho-\sigma}{\rho+\sigma}$
(c) $\sin \theta=\frac{\rho+\sigma}{\rho-\sigma}$
(d) $\sin \theta=\frac{\rho}{\sigma}$