A screw-jack has the distance between its successive threads as P and the length of its tommy bar as r. Its velocity ratio is given by the expression

• A.\(\frac{2 \pi r^2}{P}\)
• B.\(\frac{2 \pi r}{P}\)
• C.\(\frac{2 \pi}{rp}\)
• D.\(\frac{P}{2 \pi r}\)