6. For a cube wooden block $A B C D-$ $A_{1} B_{1} C_{1} D_{1}$ with an edge length of 1, take points $P, Q, R$ on the three edges passing through vertex $A_{1}$, such that $A_{1} P=A_{1} Q=A_{1} R$. After cutting off the tetrahedron $A_{1}-P Q R$, use the section $\triangle P Q R$ as the base to drill a triangular prism-shaped hole in the cube, making the sides of the prism parallel to the body diagonal $A_{1} C$. When the hole is drilled through, vertex $C$ is cut off, and the exit is a spatial polygon. How many sides does this spatial polygon have?
(A) 3
(B) 6
(C) 8
(D) 9
6. (B).

Through point $R$, a line parallel to $A_{1} C$ intersects $A C$ at a point $R^{\prime}$. Through point $Q$, a line parallel to $A_{1} C$ intersects $B_{1} C$ at a point $Q^{\prime}$. A plane through $R Q$ and parallel to $A_{1} C$ intersects the side face of the triangular prism at a point $C_{0}$ on the edge $C_{1} C$. Thus, the plane through $R Q$ and parallel to $A_{1} C$ cuts out two segments $R^{\prime} C_{0}$ and $Q^{\prime} C_{0}$ near point $C$. Similarly, planes through $P Q$ and $R P$ and parallel to $A^{\prime} C$ each cut out two segments near point $C$.
Therefore, the spatial polygon cut out at the exit has a total of 6 sides.