Simplify Assume That Variables in the Exponent Represent Nonzero Integers $x ^ { 3 a } \cdot x ^ { 6 a }$

Multiple Choice

Simplify. Assume that variables in the exponent represent nonzero integers and that all other variables are not 0.
- $x ^ { 3 a } \cdot x ^ { 6 a }$

A) $a x ^ { 18 }$
B) $a x ^ { 9 }$
C) $\mathrm { x } ^ { 18 a }$
D) $x ^ { 9 a }$