6. A line is drawn through the focus $F$ of the parabola $y=a x^{2}(a>0)$, intersecting the parabola at points $P$ and $Q$. If the lengths of segments $P F$ and $F Q$ are $p$ and $q$ respectively, then $\frac{1}{p}+\frac{1}{q}$ equals
A. $2 a$
B. $\frac{1}{2 a}$
C. $4 a$
D. $\frac{4}{a}$
6. $\mathrm{C}$ Hint: Taking the focus $F$ as the pole and the positive direction of the $y$-axis as the positive direction of the polar axis, the polar equation of the parabola is $\rho=\frac{\frac{1}{2 a}}{1-\cos \theta}, \frac{1}{\rho}=2 a(1-\cos \theta)$, Let $P(\rho, \theta), Q(q, \theta+\pi)$, then $\frac{1}{\rho}+\frac{1}{q}=2 a(1-\cos \theta)+2 a(1+\cos \theta)=$ $4 a$.