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 [IIT-JEE (Screening) 2002]

• A.            \[qb\,B/m\]                         
• B.            \[q(b-a)B/m\]
• C.            \[qa\,B/m\]                          
• D.            \[q(b+a)B/2m\]