At the same time, they all contain letters a, B, C, and the coefficient is 1 A. 4 B. 12 C. 15 D. 25

At the same time, they all contain letters a, B, C, and the coefficient is 1 A. 4 B. 12 C. 15 D. 25

a. The indexes of B and C are: 1,1,5; 1,2,4; 1,3,3; 1,4,2; 1,5,1; 2,1,4; 2,2,3; 2,4,1; 3,1,3; 3,2,2; 3,3,1; 4,1,2; 4,2,1; 5,1,1; a total of 15 cases