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. \[\frac{qbB}{m}\]         
• B. \[\frac{q(b-a)B}{m}\]
• C. \[\frac{qaB}{m}\]         
• D. \[\frac{q(b+a)B}{2m}\]