The ellipse x2+4y2=4 is inscribed in a rectangle aligned with the coordinate axes, which in turn is inscribed in another ellipse that passes through the point (4,0). Then, find the equation of the ellipse.
a. x2+12y2=16
b. 4x2+48y2=48
c. 4x2+64y2=48
d. x2+16y2=16