Practice problem

What Is the Horizontal Line Test?

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\begin{array}{|ccc|} \hline a&b&c\\ \begin{array}{|c|c|} \hline x&y\\ \hline 3&4\\ 8&7\\ 12&7\\ \hline \end{array} & \begin{array}{|c|c|} \hline x&y\\ \hline 3&4\\ 8&7\\ 12&13\\ \hline \end{array} & \begin{array}{|c|c|} \hline x&y\\ \hline 3&4\\ 8&7\\ 8&13\\ \hline \end{array}\\ \hline \end{array}

Select all of the following tables which represent $y$ as a function of $x$ and are one-to-one.