The locus of the midpoints of the chords of the circle x²+y²−ax−by=0 which subtend a right angle at (a²,b²) is
A. ax+by=0
B. ax+by=a²+b²
C. x²+y²−ax−by−a²+b²/8=0
D. x²+y²−ax−by+a²+b²/8=0