Which expression is a sum of cubes?

A sum of cubes has two terms that are each perfect cubes. In A, 64 = 4³, and the variable exponents 6 and 12 are multiples of 3; likewise, 125 = 5³ and the exponents 9 and 3 are multiples of 3. Thus both terms are cubes, so the expression is a sum of cubes.

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