17. The locus of the mid-point of the line segment joining the focus to a moving point on the parabola $y ^ { 2 } = 4 a x$ is another parabola with directrix\\
(A) $\mathrm { x } = - \mathrm { a }$\\
(B) $x = - a / 2$\\
(C) $\quad x = 0$\\
(D) $\quad x = a / 2$\\