$28 \cdot 5$ A cube is formed by welding 12 pieces of wire, each 3 inches long. A fly first lands on one vertex, then crawls along the wires. If it does not turn back and does not walk on the same path, the maximum distance it can crawl to return to the original landing point is 
(A) 24 inches.
(B) 12 inches.
(C) 30 inches.
(D) 18 inches.
(E) 36 inches.
(8th American High School Mathematics Examination, 1957)
［Solution］As shown in the figure, the fly can travel along a maximum of eight edges, so the maximum distance it can crawl is $3 \cdot 8=24$ (inches).

Otherwise, if the fly could travel along more than eight edges, then since each edge has two endpoints, the number of endpoints would be more than 16.

However, a cube has 8 vertices, and each vertex is the intersection of three edges, so there must be a vertex from which the fly has crawled along all three edges, which is impossible.
Therefore, the answer is $(A)$.