You are working on a transportation problem in Excel and have created a Solver model to find the lowest cost solution for transporting items from three distribution centers to eight wind farms. The model includes constraints to ensure that integer numbers of parts are sent and non$-$negative numbers of parts are used. What does the constraint $$integer$$ in the Solver model ensure $?$

Option A: the distance between each distribution center and wind farm is an integer value.

Option B: the total cost of transportation is an integer value.

Option C: only whole numbers of items can be sent to each wind farm.

Option D: the total demand for wind farm replacement parts is a variable.