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# A proton, a deuteron and an $\alpha$ - particle having the same kinetic energy are moving in circular trajectories in a constant magnetic field. If $r_p, r_d$ and $r_{\alpha}$ denote respectively the radii of the trajectories of these particles, then

$\begin {array} {1 1} (a)\;r_{\alpha} = r_p < r_d & \quad (b)\;r_{\alpha} = r_d < r_p \\ (c)\;r_{\alpha} = r_p = r_d & \quad (d)\;r_{\alpha} < r_p < r_d \end {array}$

$r = \sqrt{ \large\frac{(2mK)}{qB}}$
Ans : (a)