If $x , y$ satisfy the constraints $\left\{ \begin{array} { l } x + y \geqslant 2 , \\ x + 2 y \leqslant 4 , \end{array} \right.$ then the maximum value of $z = 2 x - y$ is A. $- 2$ B. 4 C. 8 D. 12
If $x , y$ satisfy the constraints $\left\{ \begin{array} { l } x + y \geqslant 2 , \\ x + 2 y \leqslant 4 , \end{array} \right.$ then the maximum value of $z = 2 x - y$ is
A. $- 2$
B. 4
C. 8
D. 12