Four particles, each of mass `1kg` are placed at the corners of a square of side `1m` in the `XY` plane. If the point of intersecting of the diagonals of the square is taken as the origin, the co-ordinates fo the centre of mass are
A. `(1,1)`
B. `(-1,1)`
C. `(1,-1)`
D. `(0,0)`