13. The dihedral angle $\alpha-l-\beta$ has a plane angle of $\varphi$, a line $m \subset \alpha, m$ forms an angle $\theta$ with the plane $\beta$, then $(\quad)$.
A. $\varphi \geqslant \theta$
B. $\varphi \leqslant \theta$
C. If and only if $\varphi \geqslant \frac{\pi}{2}$, then $\varphi \geqslant \theta$
D. If and only if $\varphi<\frac{\pi}{2}$, then $\varphi<\theta$
13. A. When $\varphi \geqslant \frac{\pi}{2}$, the conclusion holds. When $\varphi<\frac{\pi}{2}$, as shown in the figure, take any point $A$ on line $m$, draw $A C \perp$ plane $\beta$ at $C$, draw $A D \perp M N$ at $D$, connect $C D, B C$, thus $\varphi=\angle C D A, \theta=\angle A B C$, so $\sin \varphi=\frac{A C}{A D}, \sin \theta=\frac{A C}{A B}$, but $A D \leqslant A B$, hence $\sin \theta \geqslant \sin \varphi$. Since $\varphi, \theta \in\left[0, \frac{\pi}{2}\right]$, the answer is A.