3. As shown in the figure, $F_{1}, F_{2}$ are the foci of the ellipse, and the line segment $M N$ passing through $F_{2}$ has an inclination angle of $\theta$. The perimeter of $\triangle M N F_{1}$ is $P$, then ( ).
A. $P$ increases as $\theta$ increases
B. $P$ decreases as $\theta$ increases
C. $P=4\left|F_{1} F_{2}\right| \sin \theta$ 
D. The size of $P$ is independent of $\theta$
3. D. Let the major axis of the ellipse be $2 a$, then $p=4 a$, so the answer is D.