You have built a Solver model to find the solution to a transportation problem. The objective is to minimize the total cost. Constraints include linking constraints for distribution centers, ensuring integer values for the number of units moved, capacity constraints, and meeting demand for all wind farms. What is the purpose of the integer constraints in your transportation model? Option A: to ensure that only compete units are sent between cities and distribution centers. Option B: to enforce whether a distribution center should be open or closed based on the flow of units through it$.$ Option C: to limit the total number of units from each distribution centers to be less than or equal to its capacity. Option D: to make sure that no negative numbers of units are sent between cities and distribution centers.