Perform the Indicated Operation and Write Your Answer with Positive $$\frac { \mathrm { mn } ^ { - 1 } - \mathrm { nm } - 1 } { \mathrm {~m} ^ { 2 } - \mathrm { n } ^ { 2 } }$$

Multiple Choice

Perform the indicated operation and write your answer with positive integer exponents.
- $$\frac { \mathrm { mn } ^ { - 1 } - \mathrm { nm } - 1 } { \mathrm {~m} ^ { 2 } - \mathrm { n } ^ { 2 } }$$

A) $\frac { 1 } { m n }$
B) $\frac { 1 } { m + n }$
C) $m + n$
D) $\mathrm { mn }$