6. If $a, c, d$ are integers, $b$ is a positive integer, and they satisfy $a+b=c$, $b+c=d$, $c+d=a$. Then the maximum value of $a+b+c+d$ is
(A) -1 ;
(B) -5 ;
(C) 0 ;
(D) 1 .
6. (B)

Solve $(a+b)+(c+d)=c+a, \quad \therefore \quad b=-d$.
Substitute $b+c=d$ to get $c=2d, a=c+d=3d$, so $a+b+c+d=2d+3d=5d=-5b \leqslant-5(\because b \geqslant 1)$.
For example, $a=-3, b=1, c=-2, d=-1$.