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# A proton, a deuteron (nucleus of $_1 H^2$) and an $\alpha$- particle with same kinetic energy enter a region of uniform magnetic field moving at right angles to the field. The ratio of the radii of their circular paths is :

$\begin {array} {1 1} (a)\;1:2:4 & \quad (b)\;1:\sqrt 2:1 \\ (c)\;2 : \sqrt 2: 1 & \quad (d)\;1:1:2 \end {array}$

$(b) 1: \sqrt 2: 1$