Simplify the Following Expression $\frac { 5 } { 7 } - \frac { 1 } { 3 }$

Simplify the following expression. $\frac { 5 } { 7 } - \frac { 1 } { 3 }$

A) $\frac { 8 } { 21 }$
B) $\frac { 1 } { 21 }$
C) $- \frac { 8 } { 21 }$
D) $1$
E) $\frac { 7 } { 13 }$