$22 \cdot 5$ In a space, there is a finite set of points $M$ not all lying on the same plane, with the following property: for any $A \in M, B \in M$, there always exist $C \in M$ and $D \in M$ such that the lines $A B$ and $C D$ are two distinct parallel lines. The number of points in $M$ is at least
(A) 4.
(B) 6.
(C) 10.
(D) 13.
(Northeast China Mathematical Invitational Competition, 1989)
[Solution] It is easy to verify that only 4 points and 6 points in $M$ cannot satisfy the problem's requirements.
$\because 4$ points can form at most two sets of parallel lines, but 4 points can connect to $C_{4}^{2}=6$ different line segments.

Also, 6 points can form at most 6 sets of parallel lines (a regular triangular prism), but 6 points can connect to $C_{6}^{2}=15$ different line segments.
For the case of 10 points, see the figure on the right (where $M$ is the 10 vertices of a dodecahedron), which can satisfy the problem's requirements (the heights of the upper and lower quadrilateral pyramids are half the edge length of the cube).
Therefore, the answer is (C).