Your company wants to build a warehouse. Your responsibility is to suggest heating system that will be able to keep it at 15 $^\circ$C throughout the winter. You don't expect the outside temperature to drop below -3 $^\circ$C. Based on the insulative properties of the building materials, your conservative estimate for heat loss is 12 kW on the coldest day. Your company does not want the heating system to consume more than 3.9 kW. As you decide on the best system to recommend, what is the minimum COP that would be acceptable under these conditions?