AHSME 1951 Problem 17

Indicate in which one of the following equations $y$ is neither directly nor inversely proportional to $x$: $\textbf{(A)}\ x + y = 0 \textbf{(B)}\ 3xy = 10 \textbf{(C)}\ x = 5y \textbf{(D)}\ 3x + y = 10$ $\textbf{(E)}\ \frac {x}{y} = \sqrt {3}$