Let $T_f$ be the $r^{th}$ term of an A.P whose first term is 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

Question

( A ) $1$
( B ) $\frac{1}{m} + \frac{1}{n}$
( C ) $0$
( D ) $\frac{1}{mn}$