Simplify Assume That Variables Represent Positive Real Numbers $\sqrt [ 3 ] { ( - 7 ) ^ { 3 } }$

Simplify. Assume that variables represent positive real numbers.
- $\sqrt [ 3 ] { ( - 7 ) ^ { 3 } }$

A) 21
B) 7
C) $- 21$
D) $- 7$