For a flat pivot bearing of radii \(r_{1}\) and \(r_{2}\), the mean radius with uniform pressure is

(a) \(\frac{1}{2}\left(r_{1}+r_{2}\right)\)

(b) \(\frac{2}{3}\left[\frac{r_{2}^{3}-r_{1}^{3}}{r_{2}^{2}-r_{1}^{2}}\right]\)

(c) \(\frac{1}{2}\left(r_{1}-r_{2}\right)\)

(d) \(\frac{2}{3}\left[\frac{r_{2}^{3}+r_{1}^{3}}{r_{2}^{2}+r_{1}^{2}}\right]\)