Simplify the Expression $$\sqrt { \frac { 6 } { 7 } } - \sqrt { \frac { 216 } { 252 } }$$

Multiple Choice

Simplify the expression. Assume all variables represent positive real numbers.
- $$\sqrt { \frac { 6 } { 7 } } - \sqrt { \frac { 216 } { 252 } }$$

A) $- \frac { 1 } { 6 }$
B) $- 6$
C) $- 1$
D) 0