A charged particle moves at speed v and produces a magnetic field of magnitude B at a point located a distance r away, with the velocity vector perpendicular to the line joining the particle and the point. If the particle’s speed is doubled and its distance from the point is reduced to one-half, which of the following best describes the new field magnitude B′ compared with B?
a) 4B
b) 2B
c) 8B
d) 4B