A particle with mass \(m\) and positive charge \(q\) is initially at rest and accelerated through a potential difference \(\Delta V\). The particle then enters a region containing a uniform magnetic field of magnitude \(B\) oriented perpendicular to the particle's velocity vector, causing it to travel in a circular arc of radius \(r\). Which of the following expressions correctly gives the radius \(r\) in terms of the given quantities?