If the discriminant of $ax^2+2bx+c=0$ is zero, then another true statement about $a, b$, and $c$ is that: 
$\textbf{(A)}\ \text{they form an arithmetic progression}\\ \textbf{(B)}\ \text{they form a geometric progression}\\ \textbf{(C)}\ \text{they are unequal}\\ \textbf{(D)}\ \text{they are all negative numbers}\\ \textbf{(E)}\ \text{only b is negative and a and c are positive}$
The discriminant of a quadratic is
\[\Delta = b^2 - 4ac = (2b)^2 - 4ac = 4b^2 - 4ac = 0.\]
We know that $b^2 = ac,$ or $\frac{b}{a} = \frac{c}{b}$ so $a, b, c$ form a geometric progression.

$\boxed{\text{(B)}}$