A function $f$ is a rule that assigns to each element $x$ in a set $A$ exactly $(i)$ element(s) called $f(x)$ in a set $B$. Which of the following tables defines $y$ as a function of $x ?$
$$\begin{array}{|c|c|}
\hline x & y \\
\hline 1 & 5 \\
2 & 7 \\
3 & 6 \\
4 & 8 \\
\hline
\end{array}$$
$$\begin{array}{|c|c|}
\hline x & y \\
\hline 1 & 5 \\
1 & 7 \\
2 & 6 \\
3 & 8 \\
\hline
\end{array}$$