Multiply the Monomials $$\left( 4 x ^ { 5 } \right) \left( - 9 x ^ { 4 } \right)$$

Multiple Choice

Multiply the monomials.
- $$\left( 4 x ^ { 5 } \right) \left( - 9 x ^ { 4 } \right)$$

A) $36 x ^ { 9 }$
B) $- 36 x ^ { 9 }$
C) $36 x ^ { 20}$
D) $- 36 x ^ { 20 }$