Step-by-step explanation: You have two variables, the time and the number of cartons. If time wasn't a variable here, the number of cartons would remain static. You need time, otherwise there would be no change. Now that you know there are two variables, the last two choices must not be correct. Between A and B, the difference is that A says every dip is a purchase and B says every horizontal line is a purchase. Obviously, a horizontal line doesn't mean a purchase; it means the time gone WITHOUT a purchase. This leaves you with A.
