4. If each face of a tetrahedron is not an isosceles triangle, then the minimum number of edges with unequal lengths is
(A) 3 ;
(B) 4 ;
(C) 5 ;
(D) 6 .
4. (A)

Since each face of the tetrahedron is not an isosceles triangle, at least three edges have different lengths.

On the other hand, a tetrahedron with each pair of opposite edges equal can be a tetrahedron where only the lengths are all different and each face is not an isosceles triangle.