6. Given $a+b+c \neq 0$, and
$$
\frac{a+b}{c}=\frac{b+c}{a}=\frac{a+c}{b}=p \text {. }
$$

Then the line $y=p x+p$ does not pass through the ( ) quadrant.
(A) first
(B) second
(C) third
(D) fourth
6. D.

From the given information, we have
$$
\begin{array}{l}
\left\{\begin{array}{l}
a+b=c p, \\
a+c=a p, \\
c+a=b p
\end{array}\right. \\
\Rightarrow\left\{\begin{array}{l}
2(a+b+c)=p(a+b+c) \\
a+b+c \neq 0
\end{array}\right. \\
\Rightarrow p=2 .
\end{array}
$$

Therefore, the line $y=p x+p$ cannot pass through the fourth quadrant.