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. \(\frac{P}{2 \pi r}\)

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Correct Answer: B