If x2 + mx + 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 the other terms gives m = 2a and m = a², so a² = 2a; for a genuine trinomial, this gives a = 2 and m = 4. Thus x² + mx + m = x² + 4x + 4 = (x + 2)², which is choice C.

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