Simplify Write the Answer Using Positive Exponents Only $( - 3 a ) ^ { - 2 }$

Multiple Choice

Simplify. Write the answer using positive exponents only.
- $( - 3 a ) ^ { - 2 }$

A) $\frac { 1 } { 6 a ^ { 2 } }$
B) $\frac { 1 } { 9 a ^ { 2 } }$
C) $9 a ^ { 2 }$
D) $\frac { 9 } { a ^ { 2 } }$