$A$ particle with charge-to-mass ratio $\frac{q}{m} = \alpha$ is shot with a speed $v$ towards a wall at a distance $d$,moving perpendicular to the wall. The minimum value of the magnetic field $\vec{B}$ that exists in this region,perpendicular to the velocity of the particle,such that the particle does not hit the wall is:

• A
  $\frac{v}{\alpha d}$
• B
  $\frac{2v}{\alpha d}$
• C
  $\frac{v}{2\alpha d}$
• D
  $\frac{v}{4\alpha d}$