The impulse turbine rotor efficiency will have a maximum value of 0.5 cos2 α, where α is the nozzle exit flow angle, if the

A)
blades are equiangular
B)
blade solidity is 0.65
C)
blade velocity coefficient is unity
D)
blades are equiangular and frictionless

Correct Answer : Option (D) :   blades are equiangular and frictionless