Let $T_{r}$ be the $r$ th term of an A.P. whose first term is $\underline{a}$ and common difference is $d .$ If for some positive integers $m, n, m \neq n, T_{m}=\frac{1}{n}$ and $T_{n}=\frac{1}{m}$, then $a-d$,
equals
(A) 0
(B)
(C) $\frac{1}{m n}$
(D) $\frac{1}{m}+\frac{1}{n}$