If $a_{e}, a_{m}, a_{n}$ are the $\ell$ th $m$ th and $n$ th terms of an AP, then $\left|\begin{array}{lll}a_{t} & \ell & 1 \\ a_{m} & m & 1 \\ a_{n} & n & 1\end{array}\right|$ is equal to
(a) $\ell \mathrm{mn}$
(b) $\ell+\mathrm{m}+\mathrm{n}$
(c) $\ell+\mathrm{m}+\mathrm{n}-3$
(d) 0