6 . For the parabola $y^{2}=4 x$, a line is drawn through its focus intersecting the parabola at points $P, Q$. Then, the equation of the locus of the midpoint of segment $P Q$ is (
(A) $y=2 x-1$
(D) $y^{2}=2 x-2$
(C) $y'=-2 x+1$
(D) $y^{2}=-2 x+2$
G. (B)

Since the focus $F(1,0)$ should lie on the trajectory, (A) and (C) are incorrect. Also, because the parabola opens to the right, the trajectory cannot open to the left, so (D) is incorrect. Therefore, the answer is (B).