$23 \cdot 2$ 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.
(China High School Mathematics League, 1986)
[Solution] As shown in Figure (1), construct $\square A B C D$ such that $A B=a, A D=b$, and the shorter diagonal $B D=$ $c(a, b, c$ are unequal $)$.

Fix $\triangle A B D$, and fold $\triangle B C D$ along the side $B D$ until $A C=c$, resulting in the tetrahedron shown in Figure (2). Each face of this tetrahedron is not an isosceles triangle, and it has only three unequal edges. Therefore, the answer is $(A)$.