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counting and probability intermediate

Problem

A child builds towers using identically shaped cubes of different colors. How many different towers with a height cubes can the child build with red cubes, blue cubes, and green cubes? (One cube will be left out.)
(A)
(B)
(C)
(D)
Solution
Arranging eight cubes is the same as arranging the nine cubes first, and then removing the last cube. In other words, there is a one-to-one correspondence between every arrangement of nine cubes, and every actual valid arrangement. Thus, we initially get . However, we have overcounted, because the red cubes can be permuted to have the same overall arrangement, and the same applies with the blue and green cubes. Thus, we have to divide by the ways to arrange the red cubes, the ways to arrange the blue cubes, and the ways to arrange the green cubes. Thus we have different possible towers. Note: this can be written more compactly as
Final answer
D