The volume (in cubic units) of the tetrahedron with edges $\hat i+\hat j+\hat k, \hat i - \hat j+\hat k$ and $\hat i+2 \hat j-\hat k$ is

Question

$$\begin {array} {1 1} (a)\;4 & \quad (b)\;2/3 \\ (c)\;1/6 & \quad (d)\;1/3 \end {array}$$