An alternating electric field,of frequency $f$,is applied across the dees (radius $\approx R$) of a cyclotron that is being used to accelerate protons (mass $\approx m$). The operating magnetic field $(B)$ used in the cyclotron and the kinetic energy $(K)$ of the proton beam,produced by it,are given by:

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