Simplify the Radical Expression $9 \sqrt { 7 } - 3 \sqrt { 7 }$

Multiple Choice

Simplify the radical expression. Assume all variables represent positive real numbers.
- $9 \sqrt { 7 } - 3 \sqrt { 7 }$

A) $6 \sqrt { 7 }$
B) $6 \sqrt { 14 }$
C) $- 27 \sqrt { 14 }$
D) $12 \sqrt { 7 }$