The efficiency of a power screw is maximum, when the lead (helix) angle is

• A. \(\frac{{\rm{\pi }}}{2} - \frac{{\rm{\phi }}}{2}\)
• B. \(\frac{{\rm{\pi }}}{2} - {\rm{\phi }}\)
• C. \(\frac{{\rm{\pi }}}{4} - \frac{{\rm{\phi }}}{2}\)
• D. \(\frac{{{\rm{\pi \;}}}}{4} - {\rm{\phi }}\)