Maximum blade efficiency of a Parson’s turbein is given by $\alpha$ is nozzle angle)
(a) $\frac{\cos ^{2}\alpha }{1+2\cos ^{2}\alpha }$ (b) $\frac{\cos ^{2}\alpha }{1+\cos ^{2}\alpha }$
(c) $\frac{2\sin ^{2}\alpha }{1+\sin ^{2}\alpha }$ (d) $\frac{2\cos ^{2}\alpha }{1+\cos ^{2}\alpha }$