The endpoints of the longest chord on a circle are (4,5.5) and (4,10.5). The center of the circle is at point A (-4,6.75) B(4,8) C(4,9.25) D( 4,16) and its radius is A 1.25 B 2.5 C 3.75 D 6.25 units. The equation of this circle in standard form is A (x-4)^2+(y-8)^2=2.5 B (x+4)^2+(y+8)^2=2.5 C (x-4)^2(y-8)^2=6.25 D (x+4)^2(y+8)^2=6.25