Achange in refers to a movement along the demand curve for cars in response to changes in the price of cars, whereas a change in refers to a shift of the demand curve for cars leftward or rightward in response to anything other than changes in the price of cars.

quantity demanded; demand

Explanation:

The difference between demand and quantity demanded is that demand is the whole curve that relates possible prices to certain quantities, while demanded quantity means a specific point in the curve relating price to quantity. For this reason, we can conclude that  a change in quantity demanded refers to a movement along the demand curve for cars in response to changes in the price of cars, whereas a change in demand refers to a shift of the demand curve for cars leftward or rightward in response to anything other than changes in the price of cars.

answer; the changing age structure of the population;