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

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