`ABCD` is a square in first quadrant whose side is a, taking `AB and AD` as axes, prove that the equation to the circle circumscribing the square is `x^2+ y^2= a(x + y)`.
A. `x^(2)+y^(2)+ax+ay=0`
B. `x^(2)+y^(2)+ax-ay=0`
C. `x^(2)+y^(2)-ax-ay=0`
D. `x^(2)+y^(2)-ax+ay=0`