The angular displacement of the rod is defined as $\theta=\frac{3}{20} t^{2}$ where $\theta$ is in radian and $t$ is in second. The collar $B$ slides along the rod in such a way that its distance from $O$ is, $r=0.9-0.12 t^{2}$ where $r$ is in metre and $t$ is in second. The velocity of collar at $\theta=30^{\circ}$ is:
(a) $0.45 \mathrm{~m} / \mathrm{s}$
(b) $0.48 \mathrm{~m} / \mathrm{s}$
(c) $0.52 \mathrm{~m} / \mathrm{s}$
(d) $0.27 \mathrm{~m} / \mathrm{s}$