A close estimate of the radius of the earth's equator is 3,960 miles. Which of the following is the equation of the equator, if the center of the earth is considered to be the origin?
A: x^2+y^2=3,960
B:(x-3,960)^2+(y-3,960)^2=15,681,600
C:x^2+y^2=15,681,600
D:(x+3,960)^2+(y+3,960)^2=15,681,600
