由题意,得到方程组:A+C=2B --> A-2B=-C2A=B+C-6 --> 2A-B=C-6两式相加得3A-3B=-6要求的是A-B=6/3=-2答案是A
A+C=2B2A=B+C-6de 3A=3B-6所以答案是 A