If each face of a tetrahedron is not an equilateral triangle, then the minimum number of edges with unequal lengths is
(A) 3;
(B) 4 ;
(C) 5 ;
(D) 6 .
This is a question that tests students' ability to express and imagine spatial figures. A tetrahedron has six edges, and any one of these edges is a skew line with only one of the other five edges. According to the problem, only two edges that are skew lines can be of equal length. Therefore, the minimum number of edges with different lengths in the tetrahedron mentioned in the problem should be 3. Furthermore, students with a clear spatial concept can easily construct a tetrahedron whose four faces are all congruent scalene triangles, making it easier to arrive at the answer (A).