What is the completely factored form of 25x4 – 16y2?

Recognize 25x⁴ − 16y² as a difference of squares: 25x⁴ = (5x²)² and 16y² = (4y)². Apply a² − b² = (a + b)(a − b) to get (5x² + 4y)(5x² − 4y), choice C.

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