为什么答案是B？我选D

2. if for all x, x^2+ax+b=(x+c)^2, then a=?

(1)b=0,

(2)c=-3

my answer is:

if x^2+ax+b=(x+c)^2

then x^2+ax+b=x^2+2cx+c^2

so a=2c and b=c^2

(1) b=0 so c=0 so that a=2c=0

(2) a=2c=-6

所以我选d 可是答案是b