Simplify the Expression $$\sqrt { 9 x ^ { 2 } }$$

Multiple Choice

Simplify the expression.
- $$\sqrt { 9 x ^ { 2 } }$$

A) $3 x ^ { 2 }$
B) $9 x$
C) $- 3 x$
D) $3 | \mathrm { x } |$