Circle C₁ passes through the origin O and has a diameter of 4 on the positive x-axis. The line y = 2x gives a chord Oₐ of the circle C₁.
A) The radius of C₁ is 2.
B) The equation of C₁ is x² + y² = 4.
C) The coordinates of point A are (1, 2).
D) The length of chord Oₐ is 4.