A5. The first four non-zero terms of an arithmetic sequence are $a, x, b, 2x$. The ratio $a: b$ is equal to:
(A) $\frac{1}{4}$
(B) $\frac{1}{3}$
(C) $\frac{1}{2}$
(D) $\frac{2}{3}$
(E) 2
A5. If a sequence does not have all terms equal to 0 and the second and fourth terms are $x$ and $2x$ respectively, the sequence is not constant. The first four terms of the arithmetic sequence are $a, a+d=x, a+2d=b$, and $a+3d=2x$, where $d \neq 0$. From $a+d=x$ and $a+3d=2x$, we conclude that $a=d$. This means the desired ratio is $\frac{a}{b}=\frac{d}{3d}=\frac{1}{3}$.