$22 \cdot 3$ Given a rectangular parallelepiped $A B C D-A^{\prime} B^{\prime} C^{\prime} D^{\prime}$, among the following twelve lines: $A B^{\prime}, B A^{\prime}, C D^{\prime}, D C^{\prime}, A D^{\prime}, D A^{\prime}, B C^{\prime}$, $C B^{\prime}, A C, B D, A^{\prime} C^{\prime}, B^{\prime} D^{\prime}$, the number of pairs of skew lines is
(A) 30 pairs.
(B) 60 pairs.
(C) 24 pairs.
(D) 48 pairs.
[Solution] These 12 lines are precisely the 12 diagonals of the faces of the rectangular prism. For each diagonal on a face, there are five diagonals on the other five faces that are skew to it. Thus, the 12 diagonals form 60 pairs of skew diagonals (with repeated counts), but in this counting, each pair of skew lines is counted twice, so there are only 30 pairs of skew lines. $\square$
Therefore, the answer is $(A)$.