What is the center of the circle x² + y2 - 4 = 0?

(0,0)

Explanation: The equation for a circle is (x - h)^2 + (y - k)^2 = r^2, where (h,k) is the vertex and r is the radius. Rearranging the given equation, we have that x^2 + y^2 = 4. Therefore, the center of the circle is (0,0), and its radius is 2.