9.
设 $a \in \mathbf{R}$ ，函数 $f(x)=\left\{\begin{array}{ll}\cos (2 \pi x-2 \pi a) . & x
A. $\left(2, \frac{9}{4}\right] \cup\left(\frac{5}{2}, \frac{11}{4}\right]$
B. $\left(\frac{7}{4}, 2\right) \cup\left(\frac{5}{2}, \frac{11}{4}\right)$
C. $\left(2, \frac{9}{4}\right] \cup\left[\frac{11}{4}, 3\right)$
D. $\left(\frac{7}{4}, 2\right) \cup\left[\frac{11}{4}, 3\right)$