5. Given a quadrilateral pyramid $S$ $A B C D$, where the base quadrilateral $A B C D$ is not a parallelogram, using a plane to intersect the quadrilateral pyramid to get the quadrilateral $A_{1} B_{1} C_{1} D_{1}$ (outer figure), let the set $M$ $=\left\{\right.$ quadrilateral $A_{1} B_{1} C_{1} D_{1}$ is a parallelogram $\}$. Then ( ).
(A) $M$ is an empty set
(B) $M$ is an infinite set
(C) $M$ is a singleton set
(D) The elements of $M$ are unrelated to the shape of the base, and cannot be determined
(5) (B).

Let the plane $SAB$
intersect with the plane $SCD$
at the line $l_{1}^{\prime}$ passing through $S$,
the plane $SAD$ intersect with
the plane $SBC$ at the line
$t_{2}$ passing through $S$. The two intersecting
lines $l_{1}, l_{2}$ determine a
plane $\alpha$ (as shown).
When the section is parallel to $\alpha$, we have
$$
A_{1} B_{1} / / l_{1} / / C_{1} D_{1}, A_{1} D_{1} / / l_{2} / / B_{1} C_{1},
$$

thus forming $\square A_{1} B_{1} C_{1} D_{1}$. It is evident that a parallelogram section exists, and by the arbitrariness of the plane parallel to $\alpha$, (B) holds.