The center of a circle is 5 units below the origin, and the radius is 10 units. What is the equation of this circle?

A circle with center (h, k) and radius r has equation (x − h)² + (y − k)² = r². Here the center is (0, −5) and the radius is 10, so the equation is x² + (y + 5)² = 100, which is choice C.

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