$25 \cdot 20$ A square has a diagonal length of $2 \sqrt{2}$, and its two diagonals coincide with the coordinate axes. The distance from the origin to this square is
(A) 1.
(B) $\sqrt{2}$.
(C) 2.
(D) $2 \sqrt{2}$.
(Tianjin, China Junior High School Mathematics Competition, 1983)
[Solution] As shown in the right figure, it is easy to find that the side length of the square is 2. The distance from the origin $O$ to one of its sides is 1. Therefore, the answer is $(A)$