设 $f(x)=\left|x-\frac{1}{2}\right|, b_n=2 \int_0^1 f(x) \sin n \pi x d x(n=1,2, \cdots)$, 令 $S(x)=$ $\sum_{n=1}^{\infty} b_n \sin n \pi x$, 则 $S\left(-\frac{9}{4}\right)=$
$\text{A.}$ $\frac{3}{4}$.
$\text{B.}$ $\frac{1}{4}$.
$\text{C.}$ $-\frac{1}{4}$.
$\text{D.}$ $-\frac{3}{4}$.