A point P(x1,y1) is examined with respect to the circle x2+y2+2gx+2fy+c=0. Which one of the following conditions guarantees that P lies exactly on the circle?
(a) (x1+g)2+(y1+f)2=(g2+f2−c)2
(b) x12+y12+2gx1+2fy1+c>0
(c) x12+y12+2gx1+2fy1+c=0
(d) (x1+g)2+(y1+f)2=g2+f2+c