The efficiency of an impulse turbine is maximum when

Question

Given V_b= Blade speed 

V= Absolute velocity of steam entering the blade

$\alpha$  = Nozzle angle

The efficiency of an impulse turbine is maximum when

(a)V_b=0.5 V cos $\alpha$       (b)V_b= V cos $\alpha$ 

(c)V_b=0.5 V² cos $\alpha$      (D)V_b= V² cos $\alpha$