The heating system in Jacob's building has worn out and he has to get a new one. All the alternatives are gas-fired furnaces, but they vary in efficiency. Model A is leased at $500/year. It has installation charges of $500, and saves $200 a year compared with his previous system. Model B is purchased for $3600, including installation. After 10 years it will have a salvage value of $1000. It saves $500 a year compared with the old system. Model C is purchased at a total cost of $8000, half paid now, half paid in two years time. It has a salvage value of $1000 after ten years, and saves $1000/year compared with the current system. Assuming an MARR of 12% and using the IRR method, which furnace should Jacob get?