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 required so that the particle can just enter the region x > b is
Options:
(a) `(qbB)/m`
(b) `q(b-a)B/m`
(c) `(qaB)/m`
(d) `q(b+a)B/(2m)`