18. Arrange $mn$ numbers arbitrarily into a square matrix, with $m$ numbers in each row and $n$ numbers in each column. In each column, select the largest number, then from the selected $m$ numbers, choose the smallest number $a$; in each row, select the smallest number, then from the selected $n$ numbers, choose the largest number $b$. Then ( ).
(A) $ab$
(C) $a \leqslant b$
(D) $a \geqslant i$
6. (D).

If $a$ and $b$ are in the same row or column, according to the problem, we have $a \geqslant b$; if $a$ and $b$ are not in the same row or column, let the number at the intersection of the column containing $a$ and the row containing $b$ be $c$, then $a \geqslant c, c \geqslant b$, thus $a \geqslant b$.