Solve the Problem $( a - 9 ) ( a + 9 )$

Multiple Choice

Solve the problem.
- $( a - 9 ) ( a + 9 )$

A) $a ^ { 2 } - 18 a - 81$
B) $a ^ { 2 } - 18$
C) $a ^ { 2 } - 81$
D) $a ^ { 2 } + 18 a - 81$