2. If $x_{0}$ is a root of the quadratic equation $a x^{2}+b x+c=0(a \neq 0)$, then the relationship between the discriminant $\Delta=b^{2}-4 a c$ and the quadratic form $M=\left(2 a x_{0}+b\right)^{2}$ is
(A) $\Delta>M$;
(B) $\Delta=M$;
(C) $\Delta<M$;
(D) Uncertain.
2. (B)

Solution Let $x_{0}$ be a root of the equation, then $a x_{0}^{2}+b x_{0}+c=0$.
Therefore,
$$
\begin{aligned}
\left(2 a x_{0}+b\right)^{2} & =4 a^{2} x_{0}^{2}+4 a b x_{0}+b^{2} \\
& =4 a\left(a x_{0}^{2}+b x_{0}+c\right)+b^{2}-4 a c \\
& =b^{2}-4 a c
\end{aligned}
$$