8. A. Reason: The dihedral angle between two adjacent faces of a regular octahedron is $\arccos \left(-\frac{1}{3}\right)$, and the dihedral angle between two adjacent faces of a regular tetrahedron is $\arccos \frac{1}{3}$, which are complementary. When a regular tetrahedron and a regular octahedron (both with edge length $a$) share one face, the remaining three faces of the tetrahedron are the extensions in the opposite direction of the three faces of the octahedron, hence the new polyhedron has a total of 7 faces.
8. (6th "Hope Cup" Invitational Competition Question) A regular tetrahedron with edge length $a$ and a regular octahedron with edge length $a$ are combined by overlapping one face. The number of faces of the new polyhedron is ( ).
A. 7
B. 8
C. 9
D. 10