Simplify the Expression and Write the Answer Without Negative Exponents $\left( x ^ { - 7 } \right) ^ { - 3 }$

Multiple Choice

Simplify the expression and write the answer without negative exponents.
- $\left( x ^ { - 7 } \right) ^ { - 3 }$

A) $\frac { 1 } { x ^ { 10 } }$
B) $\frac { 1 } { x ^ { 21 } }$
C) $- x ^ { 10 }$
D) $x ^ { 21 }$