$2 \cdot 44$ 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.
(China Junior High School Mathematics League, 1992)
[Solution] According to the problem, we have $a x_{0}^{2}+b x_{0}+c=0$.
And $\left(2 a x_{0}+b\right)^{2}=4 a\left(a x_{0}^{2}+b x_{0}+c\right)+b^{2}-4 a c$
$$
=b^{2}-4 a c=\Delta \text {. }
$$

Therefore, the answer is $(B)$.