18.99 Connecting the midpoints of the sides of a quadrilateral, the resulting quadrilateral is a rhombus. Then the shape of the original quadrilateral is
(A) Isosceles trapezoid.
(B) Rectangle.
(C) Rhombus.
(D) None of the above conclusions is correct.
(China Guangzhou, Wuhan, Fuzhou and other five cities junior high school mathematics league, 1987)
[Solution] From the given information, we know that the two diagonals of the original quadrilateral are equal. Therefore, it could be an isosceles trapezoid, it could be a rectangle, it could also be a square, or it could be some other quadrilateral, meaning its shape is uncertain.
Thus, the answer is $(D)$.