Maximum efficiency of Parson's reaction turbine is equal to:

(a) \(\frac{\cos ^{2} \alpha}{1+2 \cos ^{2} \alpha}\)

(b) \(\frac{2 \cos ^{2} \alpha}{1+\cos ^{2} \alpha}\)

(c) \(\frac{1+2 \cos ^{2} \alpha}{\cos ^{2} \alpha}\)

(d) \(\frac{1+\cos ^{2} \alpha}{2 \cos ^{2} \alpha}\)