# Let $\bar a$ be a unit vector, $\bar b= 2 \bar i+\bar j- \bar k$ and $\bar c= \bar i +3 \bar k$. The maximum value of $[\bar a\;\bar b\;\bar c]$ is
$\begin {array} {1 1} (1)\;-1 & \quad (2)\;\sqrt {10}+\sqrt 6 \\ (3)\;\sqrt {10}- \sqrt 6 & \quad (4)\;\sqrt {59} \end {array}$