设在区间 $[a, b]$ 上 $f(x)>0, f^{\prime}(x) < 0, f^{\prime \prime}(x)>0$ ，令
$$
\begin{aligned}
S_1 & =\int_a^b f(x) \mathrm{d} x, S_2=f(b)(b-a) \\
S_3 & =\frac{1}{2}[f(a)+f(b)](b-a)
\end{aligned}
$$
则
A. $S_1 < S_2 < S_3$
B. $S_2 < S_1 < S_3$
C. $S_3 < S_1 < S_2$
D. $S_2 < S_3 < S_1$