$A$ positive charge $q$ of mass $m$ is moving along the $+x$ axis with velocity $v$. We wish to apply a uniform magnetic field $B$ for time $\Delta t$ so that the charge reverses its direction,crossing the $y$ axis at a distance $d$. Then:

• A
  $B = \frac{mv}{qd}$ and $\Delta t = \frac{\pi d}{v}$
• B
  $B = \frac{mv}{2qd}$ and $\Delta t = \frac{\pi d}{2v}$
• C
  $B = \frac{2mv}{qd}$ and $\Delta t = \frac{\pi d}{2v}$
• D
  $B = \frac{2mv}{qd}$ and $\Delta t = \frac{\pi d}{v}$