The answer is "C. 7."
The equation of a circle is (x-h)²+(y-k)²=r². r² represents the radius squared, or multiplied by itself. 7 multiplied by 7 is equal to 49, so the radius of the circle would be 7.
Formula of the circle
(x-a)² +(y-b)² = r²,
where r is a radius, (a,b) - coordinates of the center.
So,
x² + y² = 49
(x-0)² + (y-0)² = 49.
Center of this circle has coordinates (0,0).
x² + y² = 7²
r = 7
Answer is C.7.
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