Perform the Indicated Operation $12 \sqrt { 7 } - 8 \sqrt { 7 } - \sqrt { 7 }$

Perform the indicated operation. Assume that all variables represent positive numbers.
- $12 \sqrt { 7 } - 8 \sqrt { 7 } - \sqrt { 7 }$

A) $5 \sqrt { 7 }$
B) $3 \sqrt { 7 }$
C) $4 \sqrt { 7 }$
D) $- 5 \sqrt { 7 }$