Simplify Assume That Variables Represent Positive Real Numbers $\sqrt [ 6 ] { ( - 3 ) ^ { 6 } }$

Simplify. Assume that variables represent positive real numbers.
- $\sqrt [ 6 ] { ( - 3 ) ^ { 6 } }$

A) 3
B) 18
C) $- 3$
D) $- 18$