From the points (3, 4), chords are drawn to the circle `x^2+y^2-4x=0` . The locus of the midpoints of the chords is (a) `x^2+y^2-5x-4y+6=0` (b)`x^2+y^2+5x-4y+6=0` (c)`x^2+y^2-5x+4y+6=0` (d)`x^2+y^2-5x-4y-6=0`