6. The function $f(x)$ is defined on $\mathbf{R}$ as an odd function, and $f(2)$ $=0$. For any $x \in \mathbf{R}$, it holds that $f(x+4)=f(x)+f(2)$. Then $f(1998)=(\quad)$.
(A) 3996
(B) 1998
(C) 1997
(D) 0
6. (D).

Given $x=-2$, then we have $f(2)=f(-2)+f(4)$, which means $f(2) + f(2) = f(4)$.
$\therefore f(4) = 2 f(2) = 0 \Rightarrow f(x+4) = f(x)$, which means $f(x)$ is a periodic function with a period of 4.
$$
\therefore f(1998) = f(499 \times 4 + 2) = f(2) = 0.
$$