The tangent $PT$ and the normal $PN$ to the parabola $y^{2}=4ax$ at a point $P$ on it meet its axis at points $T$ and $N$, respectively. The locus of the centroid of the triangle $PTN$ is a parabola whose\\
(A) vertex is $\left(\frac{2a}{3},0\right)$\\
(B) directrix is $x=0$\\
(C) latus rectum is $\frac{2a}{3}$\\
(D) focus is $(a,0)$