$18 \cdot 1$ In a plane, if there is a point that is equidistant from the four vertices of a convex quadrilateral, then this point
(A) is inside the quadrilateral.
(B) is outside the quadrilateral.
(C) is on one side.
(D) any of the above is possible.
(2nd China Tianjin "Chinese Youth Cup" Mathematics Invitational, 1988)
［Solution］The point given in the problem is the circumcenter of the convex quadrilateral, which may be inside, outside, or on one side of the quadrilateral.
Therefore, the correct choice is $(D)$.