John Is Trying to Decide How to Divide His Time

John is trying to decide how to divide his time between his job as a stocker in the local grocery store, which pays $7 per hour for as many hours as he chooses to work, and cleaning windows for the businesses downtown. He makes $2 for every window he cleans. John is indifferent between the two tasks, and the number of windows he can clean depends on how many hours he spends cleaning in a day, as shown in the table below: $$\begin{array} { | c | c | } \hline \begin{array} { c } \text { Hours Per } \\\text { Day Cleaning } \\\text { Windows }\end{array} & \begin{array} { c } \text { Total Number } \\\text { of Windows } \\\text { Cleaned }\end{array} \\\hline 0 & 0 \\\hline 1 & 7 \\\hline 2 & 11 \\\hline 3 & 14 \\\hline 4 & 16 \\\hline 5 & 17 \\\hline\end{array}$$ What is John's opportunity cost of cleaning windows for an hour?

A) $14
B) $8
C) $7
D) $2