If $P=(1,0), Q=(-1,0)$ and $R=(2,0)$ are three given points, then locus of the point $S$ satisfying the relation $S Q^{2}+S R^{2}=2 S P^{2}$, is
[1988-2 Marks]
(a) a straight line parallel to $x$-axis
(b) a circle passing through the origin
(c) a circle with the centre at the origin
(d) a straigth line parallel to y-axis.