23. The six edges of the tetrahedron $ABCD$ are 7, 13, 18, 27, 36, and 41 units long, respectively. If $AB$ is 41 units long, then the length of $CD$ is 
(A) 7. (B) 13. (C) 18. (D) 27. (E) 36.
23
B
23. In tetrahedron $ABCD$, we look at the edge of length 7 and the two triangular faces that share this edge. In these two triangles, the difference between the other two sides must be less than 7 (otherwise, they cannot form a triangle).

Among the given numbers, only the pairs $(13,18)$ and $(36,41)$ satisfy the condition. The remaining edge of length 27 is opposite to the edge of length 7, leading to the following two scenarios:

The left figure above is not valid, because in $\triangle ABD$, $13+17<1$, which means this triangle does not exist. Only the right figure is correct, $\therefore CD=13$. Therefore, the correct choice is (B)