已知A=3a的2次方+b的2次方-c的2次方,B=-2a的2次方+3b的2次方+3c的2次方,A+B+C=0,则C等于

A = 3a2+ b2 – c2 = 0B = -2a2 – b2 + 3c2= 0代入A + B + C =03a2+ b2 – c2 -2a2 – b2 + 3c2 +C = 0合并同类项a2+ 2c2 + C = 0所C = -a2 – 2c2有4个答案供选择：
A。a2+2c2B。-a2-2c2
C。5a2+2b-4c2
D。-5a2-2b2+4c2

我做的答案是D选B