Simplify the Exponential Expression $\left( 3 x ^ { 7 } \right) \left( - 9 x ^ { 9 } \right)$

Multiple Choice

Simplify the exponential expression. Write the result using only positive exponents.
- $\left( 3 x ^ { 7 } \right) \left( - 9 x ^ { 9 } \right)$

A) $- 27 x^{ 16}$
B) $- 27 x ^ { 63 }$
C) $27 x ^ { 16 }$
D) $27 x ^ { 63 }$