Which of these is the equation of an ellipse centered at the origin?
x2+y2=1x^2 + y^2 = 1x2+y2=1
x2−y2=1x^2 - y^2 = 1x2−y2=1
y=x2y = x^2y=x2
x2+2y2=4x^2 + 2y^2 = 4x2+2y2=4