Example 2. The angle $\alpha$ formed by a straight line and a plane must be
(A) acute;
(B) a first quadrant angle;
(C) $0^{\circ} \leqslant \alpha \leqslant 90^{\circ}$,
(D) $0^{\circ}<\alpha \leqslant 90^{\circ}$.
This question is knowledge-intensive, as the angle formed by a line and a plane should include four cases: oblique intersection, perpendicular intersection, parallel, and coincident. If not considered comprehensively, it's easy to answer $(A),(D)$. However, the first quadrant angle should be $2 k \pi<\alpha<2 k \pi+\frac{\pi}{2}$, so the correct choice should be $(C)$.