4. Among all the triangles formed by any 3 of the 8 vertices of a rectangular cuboid, the number of acute triangles is
(A) 0 ;
(B) 6 ;
(C) 8 ;
(D) 24 .
4. (C)

Among the triangles formed by any 3 of the 8 vertices of a rectangular cuboid, there are $C_{8}^{3}$ triangles, and these triangles can only be right-angled triangles or acute-angled triangles. Each vertex of the cuboid serves as the right angle vertex for 6 triangles, so there are a total of $6 \times 8=48$ (triangles) right-angled triangles among the $C_{8}^{3}$ triangles. Therefore, the number of acute-angled triangles is $C_{8}^{3}-48=8$.