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