Locus of the middle points of chords of the circle `x^2 + y^2 = 16` which subtend a right angle at the centre is

The locus of the midpoints of chords of the circle x^(2) + y^(2) = 1 which subtends a right angle at the origin is

The locus of the mid-points of chords of the circle x^2+y^2 =4 , which subtend a right angle at the origin, is:

The locus of the midpoint of the chord of the circle x^2 + y^2 =4 which subtends a right angle at the origin is?

The locus of the midpoint of a chord of the circle x^(2)+y^(2) = 4 which subtends a right angle at the origin is