# The vectors $\overrightarrow{a}=3\hat i+2\hat j+2\hat k$ and $\overrightarrow{b}=-\hat i+2\hat k$ are the adjacent sides of a parallelogram .The acute angle between its diagonals is__________.
$\begin{array}{1 1}(A)\;\sin ^{-1} \bigg(\sqrt {\large\frac{84}{85}}\bigg) \\(B)\; \cos^{-1}\large\frac{1}{\sqrt{85}} \\(C)\;\cos ^{-1} \bigg(\sqrt {\large\frac{85}{84}}\bigg) \\(D)\; \sin^{-1}\large\frac{4}{\sqrt{30}} \end{array}$