A grinding machine whose wheel has a radius of $\frac{1}{\mathrm{\pi }}$ is rotating at 2.5 rev/sec. A tool to be sharpened is held against the wheel with a force of 40N. If the coefficient of friction between the tool and the wheel is 0.2, power required is

• A

  40 W

• B

  4 W

• C

  8 W

• D

  10 W