If $P$ represent $z= x + iy$ in the argand plane and $|z-1|^2 + |z+1|^2 = 4$, then the locus of $P$ is :

Question

( A ) $x+y=2$
( B ) $x^2+y^2=2$
( C ) $x^2+y^2=4$
( D ) $x^2+y^2=1$