Right, lets set up some sitations.

A and B can not be the same person, so we don't need to worry about that.

1. A is in the same row as B:

If A is in the same row as B, then A is the tallest person in the row, and thus is taller than B.

2. A is in the same column as B:

If both A and B are in the same column then B is the shortest person in that column and thus is shorter than A.

3. A and B are in different columns/rows:

Imagine a third person, person "C", who is in the same row as A and the same column as B. C must be shorter than A, since A is the tallest person in the row. C must be taller than B, since B is the shortest person in the column. From this A > C > B, or A > B

So....... A is taller.