From the graph that we have here, the event that is responsible for this change is A product becomes less popular and fewer customers purchase it.
The reason this is the answer is because when e look at the graph closely, we can see that only demand fell from 50 to 40. There was no change in price, it remained at 15 dollars.
A fall in demand even at the same price that a good was sold previously shows us that less people are buying the product. This could be due to changing consumer preference.
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