If x squared plus m x plus m is a perfect-square trinomial, which equation must be true?

A perfect-square trinomial with leading term x² has the form (x + a)² = x² + 2ax + a². Matching coefficients with x² + mx + m gives m = 2a and m = a², so a² = 2a. The nonzero solution is a = 2, giving m = 4 and x² + mx + m = (x + 2)², choice D.

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