Which expression does not factor?

The sum and difference of cubes factor: m³ + 1 = (m + 1)(m² − m + 1) and m³ − 1 = (m − 1)(m² + m + 1). Also, m² − 1 = (m − 1)(m + 1). But m² + 1 is a sum of squares and does not factor into real linear factors, so the correct choice is C, m² + 1.

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