3. If $f(x)$ is an odd function with a period of 5 on $\mathbf{R}$, and satisfies $f(7)=9$, then $f(2020)-f(2018)=(\quad$ ).
A: 6
B: 7
C: 8
D: 9
Answer D.
Analysis Since $f(x)$ is an odd function with a period of 5 on $\mathbf{R}$, we have
$$
f(2020)-f(2018)=f(5 \times 404+0)-f(5 \times 404-2)=f(0)+f(2)=0+f(7)=0+9=9
$$