This question is for me. I just give a personal answer from my own feelings and understanding, and there is no theoretical basis.
If it happens to help or enlighten you, if this idea is unreasonable, please don't laugh.
I think this warehouse should be built in D city.
My reason is to take two cities as examples. The straight line distance between the two cities is at any point on this straight line, and the warehouse is transported to the two cities respectively. Under the same unit freight rate, the closer the warehouse is to the city with high demand, the lower the total freight rate. The linear distance between the two cities will not increase or decrease because of the location of the warehouse, so the warehouse is located at one end, that is, the city with high demand, so are the five cities with the lowest total freight.
So according to my above understanding, I choose D city to open a warehouse.