$19 P Q$ is any chord passing through the focus of a given parabola, $M, N$ are the projections of $P, Q$ on the directrix $l$, respectively. The lateral surface area of the solid of revolution obtained by rotating the broken line $M P Q N$ around $l$ is $S_{1}$, and the surface area of the sphere with diameter $P Q$ is $S_{2}$. Then we have
A. $S_{1}-S_{2}>0$
B. $S_{1}: S_{2}<1$
C. $S_{1}-S_{2}$ is a constant
D. $S_{1} \cdot S_{2}$ is a constant
19 C. Let the focus be $F$, then $P M + Q N = P F = F Q = P Q$. $S_{1} = \pi (P M + Q N) \cdot P Q = \pi \cdot P Q^{2}, S_{2} = 4 \pi \left(\frac{P Q}{2}\right)^{2} = \pi \cdot P Q^{2}$. Therefore, $S_{1} = S_{2}$.