"GEOMETRY QUESTION OF THE DAY'

On the xy-coordinate plane, points A and B both lie on the circumference of a circle whose center is O, and the length of AB equals the circle's diameter. If the (x,y) coordinates of O are (2,1) and the (x,y) coordinates of B are (4,6), what are the (x,y) coordinates of A?

1. (3, 3/2)
2. (1, 2/2)
3. (0, -4)
4. (2/2, 1)
5. (-1, -2/2)