5. Let $t$ be a real 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 $M=(2 a t+b)^{2}$ is ( ).
(A) $\Delta>M$
(B) $\Delta=M$.
(C) $\Delta<M$
(D) Cannot be determined

直接输出翻译结果。
5. B (Hint: Since $t$ is a root of the equation, we have $a t^{2}+b t+c=0$. Thus, $M=4 a^{2} t^{2}+4 a b t+b^{2}=4 a\left[a t^{2}+b t+c\right]-4 a c$ $+b^{2}=b^{2}-4 a c=\Delta.$)