6. If $2 x+5 y+4 z=6,3 x+y-7 z=-4$, then the value of $x+y-z$ is ( ).
(A) -1
(B) 0
(C) 1
(D) 4
6. B.
$$
\begin{array}{l}
\text { From }\left\{\begin{array}{l}
2(x+y-z)+3(y+2 z)=6, \\
3(x+y-z)-2(y+2 z)=-4
\end{array}\right. \text { we get } \\
x+y-z=0 .
\end{array}
$$