6. Given $\sin \alpha>\sin \beta$, and $\alpha \neq k \pi+\frac{\pi}{2}, \beta \neq k \pi+\frac{\pi}{2},(\quad)$.
A. If $\alpha, \beta$ are angles in the first quadrant, then $\cos \alpha>\cos \beta$
B. If $\alpha, \beta$ are angles in the second quadrant, then $\tan \alpha>\tan \beta$
C. If $\alpha, \beta$ are angles in the third quadrant, then $\cos \alpha>\cos \beta$
D. If $\alpha, \beta$ are angles in the fourth quadrant, then $\tan \alpha>\tan \beta$
6. D.

If $\alpha, \beta$ are in the fourth quadrant, $\alpha=2 k \pi-\alpha^{\prime}, \beta=2 m \pi-\beta^{\prime}, k, m \in \mathbf{Z}, \alpha^{\prime}, \beta^{\prime}$ are both acute angles, $\sin \alpha=$ $-\sin \alpha^{\prime}>-\sin \beta^{\prime}=\sin \beta$, then $\alpha^{\prime}\tan \beta$,