A force $\overrightarrow{F} = \alpha \hat{i} + 3 \hat {j} + 6 \hat{k}$ is acting at a point $\overrightarrow{r} = 2 \hat{i} -6\hat {j} - 12 \hat{k}$. The value of $\alpha$ for which angular momentum about origin is conserved is :

Question

( A ) 1
( B ) zero
( C ) 2
( D ) -1