Which choice is equivalent to the expression below? \[ \sqrt{-81} \] A. 9 i B. -9 C. \sqrt{9 i} D. -\sqrt{9} E. i \sqrt{9}

Step 1 ：The square root of a negative number is an imaginary number.

Step 2 ：The square root of -81 can be written as the square root of 81 times the square root of -1.

Step 3 ：The square root of 81 is 9 and the square root of -1 is represented as 'i' in mathematics.

Step 4 ：Therefore, the square root of -81 is 9i.

Step 5 ：The equivalent expression to \(\sqrt{-81}\) is \(\boxed{9i}\).