A proton and an $\alpha$-particle (with their masses in the ratio of $1 : 4$ and charges in the ratio of $1 : 2$) are accelerated from rest through a potential difference $V$. If a uniform magnetic field $(B)$ is set up perpendicular to their velocities, the ratio of the radii $r _ { p } : r _ { \alpha }$ of the circular paths described by them will be:\\
(1) $1 : 3$\\
(2) $1 : \sqrt { 2 }$\\
(3) $1 : 2$\\
(4) $1 : \sqrt { 3 }$