Two uncharged parallel plates A and B are at the same electrical potential, and separated by a distance d. A uniform magnetic induction field B acts perpendicular to plane of paper (and parallel to plates) between the plates. Particles, each with charge q, mass m and kinetic energy
K are shot through a hole in A directly towards B, particle entering the hole perpendicular to plate A. An ammeter A_m connected to B and measures the current collected by the plate B, as the particles hit it. The limiting value of the field B for which the ammeter will stop showing a current is

• A

  $\frac{\sqrt{2\mathrm{mK}}}{\mathrm{qd}}$

• B

  $\frac{\sqrt{3\mathrm{mK}}}{\mathrm{qd}}$

• C

  $\frac{\sqrt{\mathrm{mK}}}{2\mathrm{qd}}$

• D

  $\frac{2\sqrt{\mathrm{mK}}}{\mathrm{qd}}$