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