Which expression is a perfect cube?

A monomial is a perfect cube when its coefficient is an integer cube and each variable’s exponent is divisible by 3. In choice A, −216 = (−6)³, and both 9 and 18 are divisible by 3, so the expression is (−6x³y⁶)³. The other choices have at least one variable exponent that is not divisible by 3.

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