5. In a regular triangular prism $A B C-A_{1} B_{1} C_{1}$, among the six diagonal lines on the lateral faces $A B_{1} 、 A_{1} B 、 B C_{1} 、 B_{1} C 、 C_{1} A 、 C A_{1}$, if one pair $A B_{1} \perp B C_{1}$, how many other pairs, denoted as $k$, are also perpendicular to each other? Then $k$ equals ( ).
(A)2
(B) 3
(C) 4
(D) 5
5.D.

As shown in Figure 1, take the midpoint $M$ of $B C$, and connect $B_{1} M$. Then the projection of $A B_{1}$ on the plane $B B_{1} C_{1} C$ is $B_{1} M$. Given that $A B_{1} \perp B C_{1}$, we know that $B_{1} M \perp B C_{1}$.

Next, take the midpoint $N$ of $B_{1} C_{1}$, we have $C N \perp B C_{1}$, thus $A_{1} C \perp B C_{1}$. 
From the fact that the side faces of the regular triangular prism are congruent rectangles, we can get $A B_{1} \perp A_{1} C, A_{1} B \perp B_{1} C, A_{1} B \perp A C_{1}, B_{1} C \perp A C_{1}$. Therefore, there are 5 pairs.