A person purchased 10 items to donate to a school. They bought backpacks that cost $25.75 each and notebooks that cost $4.25 each. The total cost of the items is $150. This situation can be represented by a system of equations with a unique solution. What does the solution (x, y) to the system of equations represent in this context?

the cost of all the backpacks

the cost of all the notebooks

the total amount of money spent on the items

the number of backpacks and notebooks purchased