6. Among the 8 vertices of a cube, the number of right-angled triangle groups formed by 3 vertices is ( ).
(A) 36
(B) 37
(C) 48
(D) 49
6. (C).

As shown in Figure 3, note that each square or rectangle has 4 "right-angle triplets." There are a total of 6 side faces and 6 diagonal sections, so there are $4 \times(6+6)=48$ "right-angle triplets."