2. As shown in the figure, in the regular tetrahedron $ABCD$, $E$ is on edge $AB$, and $F$ is on edge $CD$, such that $\frac{AE}{EB} = \frac{CF}{FD} = \lambda (0 < \lambda < +\infty)$. Let $f(\lambda) = \alpha_{\lambda} + \beta_{\lambda}$, where $\alpha_{\lambda}$ is the angle between $EF$ and $AC$, and $\beta_{\lambda}$ is the angle between $EF$ and $BD$. Then,
(A) $f(\lambda)$ is monotonically increasing on $(0, +\infty)$
(B) $f(\lambda)$ is monotonically decreasing on $(0, +\infty)$
(C) $f(\lambda)$ is monotonically increasing on $(0, 1)$ and monotonically decreasing on $(1, +\infty)$
(D) $f(\lambda)$ is a constant on $(0, +\infty)$

Answer $(\quad)$
(D)
2. 【Analysis and Solution】Since $A B C D$ is a regular tetrahedron, $A C \perp B D$. Draw $B G / / A C$ intersecting $B C$ at $G$, and connect $G F$, then $\alpha_{\lambda}=$
$$
\angle E F G+\angle E F G=180^{\circ}-\angle E G F=90^{\circ} \text {. }
$$