John Is Trying to Decide How to Divide His Time

Multiple Choice

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.  
The first hour John spends cleaning windows costs him ________ that he could have earned in the grocery store.

A) $2
B) $7
C) $14
D) $18