6 The curve represented by the equation $\frac{x^{2}}{\sin \left(19^{n}\right)^{\circ}}+\frac{y^{2}}{\cos \left(19^{n}\right)^{\circ}}=1\left(n \in \mathbf{N}^{*}\right)$ is
(A) a hyperbola with foci on the $x$-axis
(B) a hyperbola, the axis on which its foci lie depends on $n$
(C) an ellipse with foci on the $y$-axis
(D) an ellipse, the axis on which its foci lie depends on $n$
6) $\mathrm{C}$ Hint: When $n=1$, $\sin 19^{\circ}<\cos 19^{\circ}$; when $n=2$,
$$
\sin \left(19^{2}\right)^{\circ}=\sin 361^{\circ}=\sin 1^{\circ}<\cos 1^{\circ} ;
$$
when $n=2 k\left(k \in \mathbf{N}^{*}\right)$,
$$
\sin \left(19^{2 k}\right)^{\circ}=\sin \left(361^{k}\right)^{\circ}=\sin 1^{\circ}<\cos 1^{\circ} ;
$$
when $n=2 k+1\left(k \in \mathbf{N}^{*}\right)$,
$$
\sin \left(19^{2 k+1}\right)^{\circ}=\sin \left[19 \times(361)^{k}\right]^{\circ}=\sin 19^{\circ}<\cos 19^{\circ} \text {. }
$$

Therefore, the equation represents an ellipse with its foci on the $y$-axis. Choose C.