There are eight monomials with coefficients of 1 and letters a, B, C, D

There are eight monomials with coefficients of 1 and letters a, B, C, D

1. They all contain the letters a, B, C and D
2. And the coefficient is 1
3. Binomial of degree 8
The monomials satisfying the above three conditions are as follows:
(1) The indices are all two; there is one
(2) The index is two ones, two three; there are four
(3) The index is three ones and one five; there are four,
(4) The index is one 1, two 2, and one 3; there are 10,
(5) The index is a 4, two 1s and a 2; there are 10,
So there are such monomials: 10 + 10 + 4 + 4 + 1 = 29