6. Given positive integers $x_{1}, x_{2}, \cdots, x_{9}$ satisfying $x_{1}<x_{2}$ $<\cdots<x_{9}$, and $x_{1}^{2}+x_{2}^{2}+\cdots+x_{9}^{2} \leqslant 675$. Then the maximum value of $x_{7}-$ $x_{4}$ is ( ).
(A) 6
(B) 7
(C) 8
(D) 9
6. C.

Optimal solution:
$$
\begin{array}{l}
x_{1}=1, x_{2}=2, x_{3}=3, x_{4}=4, x_{5}=5, x_{6}=6, \\
x_{9}=15, x_{8}=14, x_{7}=13 .
\end{array}
$$

However, at this point $x_{1}^{2}+x_{2}^{2}+\cdots+x_{9}^{2}=681$ (which does not meet the requirement), so $x_{7}-x_{4}<9$.

And $1^{2}+2^{2}+\cdots+6^{2}+12^{2}+14^{2}+15^{2}=656<675$, hence the maximum value of $x_{7}-x_{4}$ is 8.