2. 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
2. B According to the problem, $a+b+c+d=a+c$, so $b+d=0, b=-d$, thus $c=2d, a=3d, a+b+c+d=$ $5d=-5b$, and $b \geqslant 1$, therefore the maximum value of $a+b+c+d$ is -5.