Simplify Assume That All Variables Represent Any Real Number $\sqrt { 81 x ^ { 2 } }$

Multiple Choice

Simplify. Assume that all variables represent any real number.
- $\sqrt { 81 x ^ { 2 } }$

A) $9 | \times |$
B) $9 x$
C) $- 9 | x |$
D) $- 9 x$