6. The function $F(x)=\cos ^{2} x+2 \sin 2 \cdot \cos x$ $-\sin ^{2} x+A x+B \mid$ has a maximum value $M$ on $0 \leqslant x \leqslant \frac{3 \pi}{2}$ that depends on the parameters $A$ and $B$. When $M$ is minimized, the values of $A$ and $B$ are ( . . .
(A) $A=0, B=1$
(B) $A=1, B=0$
(C) $A=B=0$
(D) $A=B=1$
6. (C).

When $A=B=0$, $F(x)=\sqrt{2}\left|\sin \left(2 x+\frac{\pi}{4}\right)\right|$ has three points, $x=\frac{\pi}{8}, \frac{5 \pi}{8}, \frac{9 \pi}{8}$, in $\left[0, \frac{3 \pi}{2}\right]$, where $F(x)$ reaches its maximum value $\sqrt{2}$.
When $A$ and $B$ are not both zero, $\max _{1 \leqslant x \leqslant \frac{3 \pi}{2}} F(x)>\sqrt{2}$.