If \[\vec{A}\] and \[\vec{B}\] are perpendicular vectors and vector \[\vec{A}=5\hat{i}+7\hat{j}-3\hat{k}\] and \[\vec{B}=2\hat{i}+2\hat{j}-a\hat{k}.\] The value of a is [EAMCET 1991]

• A.                 ? 2         
• B.                 8
• C.                 ? 7         
• D.                 ? 8