Answer: The change in the price of milk
Step-by-step explanation:
According to the law of demand, people will demand more of a good if the price is lower as opposed to if the price is higher. When prices change therefore it is expected that the quantity demanded of the good in question will change as well in an inverse manner.
As the price of milk increased by a dollar in this scenario, the quantity demanded of milk will most certainly change and in all likelihood for the worse because it is now more expensive.