$23 \cdot 36$ If a polyhedron has two faces that are parallel to each other, and all other faces are parallelograms, then this polyhedron is
(A) a regular prism.
(B) a right prism.
(C) a prism.
(D) may not be a prism.
(E) cannot be a prism.
(China Beijing High School Mathematics Competition, 1984)
[Solution] Obviously, a prism satisfies the conditions of the problem, but a polyhedron that meets the conditions is not necessarily a prism. The polyhedron shown in the figure satisfies having two faces parallel to each other, and all other faces are parallelograms, but it is not a prism. Therefore, the conclusion "may not be a prism" is chosen.
Hence, the answer is $(D)$.