# If the vector $\overrightarrow{a} =2 \hat i+3 \hat j+6 \hat k$ and $\overrightarrow {b}$ are collinear and $|\overrightarrow {b}|=21,$ then $\overrightarrow {b}$ is equal to :
$\begin {array} {1 1} (a)\;\pm(2 \hat i+3 \hat j+6 \hat k) & \quad (b)\;\pm3(2\hat i+3 \hat j+6 \hat k) \\ (c)\;( \hat i+ \hat j+ \hat k) & \quad (d)\;\pm 21(2 \hat i+3 \hat j+6 \hat k) \end {array}$