If α is the angle of blade tip at outlet, then maximum hydraulic efficiency of an impulse turbine is

A

$$\frac{(1 + \cos \alpha)}{2}$$

B

$$\frac{(1 - \cos \alpha)}{2}$$

C

$$\frac{(1 - \sin \alpha)}{2}$$

D

$$\frac{(1 + \sin \alpha)}{2}$$