6. Given that the function $y=f(x)$ is decreasing on $(-3,0)$, and the graph of $y=f(x-3)$ has a line of symmetry at $x=0$, which of the following conclusions is correct? ( ).
A. $f\left(-\frac{3}{2}\right)<f\left(-\frac{7}{2}\right)<f(-5)$
B. $f(-5)<f\left(-\frac{7}{2}\right)<f\left(-\frac{3}{2}\right)$
C. $f(-5)<f\left(-\frac{3}{2}\right)<f\left(-\frac{7}{2}\right)$
D. $f\left(-\frac{7}{2}\right)<f\left(-\frac{3}{2}\right)<f(-5)$
6. C. From the problem, we know that the axis of symmetry of $y=f(x)$ is the line $x=-3$. Combining this with the monotonicity, we can solve.