设 $f(x)=\left|x-\frac{1}{2}\right|, b_{n}=2 \int_{0}^{1} f(x) \sin n \pi x \mathrm{~d} x(n=1,2, \cdots)$. 令 $S(x)=\sum_{n=1}^{\infty} b_{n} \sin n \pi x$, 则 $S\left(-\frac{9}{4}\right)=$ ( )
A. $\frac{3}{4}$.
B. $\frac{1}{4}$.
C. $-\frac{1}{4}$.
D. $-\frac{3}{4}$.