An alternating electric field of frequency $f$ is applied across the radius $R$ of a cyclotron to accelerate protons (mass $m$). The operating magnetic field $B$ used and kinetic energy $(K.E.)$ of the proton beam produced by it are respectively ($e=$ charge on proton).

• A
  $\frac{2 \pi m f}{e}, 2 \pi^2 m f^2 R^2$
• B
  $\frac{2 \pi^2 m f}{e^2}, 4 \pi^2 m f^2 R^2$
• C
  $\frac{\pi m f}{e}, \pi^2 m f^2 R^2$
• D
  $\frac{2 \pi^2 m^2 f^2}{e}, 2 \pi^2 m^2 f^2 R^2$