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 accompanying table. 
If we plot John's opportunity cost per window on the vertical axis and the number of windows cleaned each day on the horizontal axis, we will have John's ________ curve for window-cleaning services.
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