The angle between the planes <br> $\overrightarrow{r} ( 2 \hat{i} - \hat{j} + 2 \hat{k}) = 3$ and <br> $\overrightarrow{r}(3 \hat{i} - 6 \hat{j} + 2 \hat{k} )= 4$ is :

Question

( A ) $\sin^{-1} (\frac{4}{21})$
( B ) $\cos^{-1} (\frac{3}{4})$
( C ) $\cos^{-1} (\frac{1}{4})$
( D ) $\cos^{-1} (\frac{4}{21})$