1955 AHSME Problem 32

If the discriminant of ax^2+2bx+c=0 is zero, then another true statement about a, b, and c is that:

\begin{array}{l} \textbf{(A)}\ \text{they form an arithmetic progression}\\ \textbf{(B)}\ \text{they form a geometric progression}\\ \textbf{(C)}\ \text{they are unequal}\\ \textbf{(D)}\ \text{they are all negative numbers}\\ \textbf{(E)}\ \text{only b is negative and a and c are positive} \end{array}

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.