The energy contents, unit costs, and typical conversion efficiencies of various energy sources for use in water heaters are given as follows: 1025 Btu/ft3, $0.012/ft3, and 85 percent for natural gas; 138,700 Btu/gal, $2.2/gal, and 75 percent for heating oil; and 1 kWh/kWh, $0.11/kWh, and 90 percent for electric heaters, respectively. Determine the lowest-cost energy source for water heaters.