Add Do Not Use a Number Line Except as a Check

Multiple Choice

Add. Do not use a number line except as a check.
- $$- \frac { 1 } { 14 } + \frac { 8 } { 21 }$$

A) $\frac { 11 } { 21 }$
B) $\frac { 1 } { 3 }$
C) $\frac { 13 } { 42 }$
D) $\frac { 13 } { 21 }$