6. Given the complex number $z=1-\sin \theta+\mathrm{i} \cos \theta\left(\frac{\pi}{2}<\theta<\pi\right)$, then the principal value of the argument of the conjugate complex number $\bar{z}$ is
A. $\frac{3 \pi}{4}-\frac{\theta}{2}$
B. $\frac{5 \pi}{4}-\frac{\theta}{2}$
C. $\frac{\pi}{2}+\frac{\theta}{2}$
D. $\frac{\theta}{2}-\frac{\pi}{4}$
6. $\mathrm{A}$