$23 \cdot 1$ tetrahedron $A B C D$ has six edges of lengths 7, 13, 18, 27, 36, and 41 units, respectively. If $A B$ is 41 units long, then the length of $C D$ is
(A) 7 .
(B) 13 .
(C) 18 .
(D) 27 .
(E) 36 .
［Solution］From the triangle inequality condition:
one side must be less than the sum of the other two sides, so these edge lengths can only be combined in the manner shown in the right figure.
Therefore, the correct choice is $(B)$.