7. For the equation $\frac{x^{2}}{\sin \alpha}+\frac{y^{2}}{\cos \alpha}=\tan \alpha(\alpha$ is a constant and $\left.\alpha \neq \frac{k \pi}{2}, k \in \mathbf{Z}\right)$, which of the following conclusions is incorrect? ( ).
(A) It can represent a hyperbola
(B) It can represent an ellipse
(C) It can represent a circle
(D) It can represent a line
7.D.

When $\alpha$ is in the first quadrant and $\alpha \neq 2 k \pi+\frac{\pi}{4}(k \in Z)$, the equation can represent an ellipse;
When $\alpha=2 k \pi+\frac{\pi}{4}(k \in Z)$, the equation represents a circle;
When $\alpha$ is in the second or fourth quadrant, the equation can represent a hyperbola;
When $\alpha$ is in the third quadrant, there is no trajectory.