A particle of mass m and charge q moves with a constant velocity v along the positive x direction' It enters a region containing a uniform magnetic field B directed along the negative Z-direction, extending from x = a to x = b. The minimum value of v required so that the particle can just enter the region x >b is

• A

  $\frac{\mathrm{qbB}}{\mathrm{m}}$

• B

  $\frac{\mathrm{q}(\mathrm{b}-\mathrm{a})\mathrm{B}}{\mathrm{m}}$

• C

  $\frac{\mathrm{qaB}}{\mathrm{m}}$

• D

  $\frac{\mathrm{q}(\mathrm{b}+\mathrm{a})\mathrm{B}}{2\mathrm{m}}$