Determine whether the tables in Exercises 11 through 18 represent functions or not. Assume that the input is in the left column or top row. Explain your answer.
$$\begin{array}{|c|c|}\hline \text { Month } & \begin{array}{c}\text { Cost } \\\text { (in dollars) }\end{array} \\\hline \text { Jan. } & 5689.35 \\\text { Feb. } & 7856.12 \\\text { May } & 2689.15 \\\text { June } & 1005.36 \\\hline\end{array}$$