6. Given that the domain of the function $f(x)$ is $[-2,+\infty)$, and $f(4)=f(-2)=$ 1. $f^{\prime}(x)$ is the derivative of $f(x)$, and the graph of the function $y=f^{\prime}(x)$ is shown in the figure on the right. Then the area of the plane region $\left\{\begin{array}{l}a \geqslant 0, \\ b \geqslant 0, \\ f(2 a+b)<1\end{array}\right.$ is
A. 2
B. 4
C. 5
D. 8