In a parson's reaction turbine, when α1 is the angle with the direction of motion of the blade at which steams enters the blade, then the maximum efficiency of the turbine is given by

• A. \(\frac{{2\cos \alpha_1 }}{{1 + \cos \alpha_1 }}\)
• B. \(\frac{{2{{\cos }^2}\alpha_1 }}{{1 + {{\cos }^2}\alpha_1 }}\)
• C. \(\frac{{2{{\cos }^2}\alpha_1 }}{{1 - \cos \alpha_1 }}\)
• D. \(\frac{{2{{\cos }^2}\alpha_1 }}{{1 + 2{{\cos }^2}\alpha_1 }}\)