6. Under the constraint conditions $\left\{\begin{array}{l}x \geqslant 0 \\ y \geqslant 0 \\ x+y \leqslant s \\ y+2 x \leqslant 4\end{array}\right.$, when $3 \leqslant s \leqslant 5$, the range of the maximum value of the objective function $z=3 x+2 y$ is
A. $[6,15]$
B. $[7,15]$
C. $[6,8]$
D. $[7,8]$
6.  From $\left\{\begin{array}{l}x+y=s \\ y+2 x=4\end{array} \Rightarrow\left\{\begin{array}{l}x=4-s \\ y=2 s-4\end{array}\right.\right.$. The intersection points are $A(0,2), B(4-s, 2 s-4)$. $C(0, s), C^{\prime}(0,4)$
(1) When $3 \leqslant s<4$, the feasible region is quadrilateral $O A B C$, at this time, $7 \leqslant z \leqslant 8$.
(2) When $4 \leqslant s \leqslant 5$, the feasible region is $\triangle O A C^{\prime}$, at this time, $z_{\max }=8$, hence the answer is D.