AHSME 1950 Problem 41

The least value of the function $ax^2 + bx + c$ with $a>0$ is: $\textbf{(A)}\ -\dfrac{b}{a} \textbf{(B)}\ -\dfrac{b}{2a} \textbf{(C)}\ b^2-4ac \textbf{(D)}\ \dfrac{4ac-b^2}{4a} \textbf{(E)}\ \text{None of these}$