6. As shown in Figure 2, in the regular pentagon $A B C D E$, the diagonals intersect at points $A_{1} 、 B_{1} 、 C_{1} 、 D_{1} 、 E_{1}$. All congruent triangles are considered as one class, called a "congruence class" (for example, $\triangle A B C 、 \triangle B C D$ and $\triangle C D E$ all belong to the same congruence class). The number of different congruence classes in Figure 2 is ( ).
(A) 3
(B) 4
(C) 5
(D) 6
6. C.

In Figure 2, there are no triangles with all three vertices being from $A_{1}, B_{1}, C_{1}, D_{1}, E_{1}$.
(1) The congruence classes of triangles with all vertices taken from $A, B, C, D, E$ are 2: the $\triangle ABC$ congruence class and the $\triangle ABD$ congruence class;
(2) The congruence classes of triangles with two vertices taken from $A, B, C, D, E$ and one vertex taken from $A_{1}, B_{1}, C_{1}, D_{1}, E_{1}$ are 3: the $\triangle ABA_{1}$ congruence class, the $\triangle ABB_{1}$ congruence class, and the $\triangle ACD_{1}$ congruence class;
(3) The congruence class of triangles with one vertex taken from $A, B, C, D, E$ and two vertices taken from $A_{1}, B_{1}, C_{1}, D_{1}, E_{1}$ is 1: the $\triangle BA_{1}B_{1}$ congruence class;
However, $\triangle ABC$ and $\triangle ACD_{1}$ belong to the same congruence class. Therefore, there are a total of 5 different congruence classes.