Rewrite the Expression Using Only Positive Exponents, and Simplify $\frac { s ^ { - 1 } + r ^ { - 1 } } { s ^ { - 1 } - r ^ { - 1 } }$

Multiple Choice

Rewrite the expression using only positive exponents, and simplify. Assume that any variables in the expression are nonzero. $\frac { s ^ { - 1 } + r ^ { - 1 } } { s ^ { - 1 } - r ^ { - 1 } }$

A) $\frac { r - s } { r + s }$
B) $s ^ { 2 } + r ^ { 2 }$
C) $\frac { r + s } { r - s }$
D) $\frac { s + r } { s - r }$
E) $\frac { s - r } { s + r }$