Reconsider Prob. 2–60E Using EES (or other) software, study the effects of the unit cost of energy and combustion efficiency on the annual energy used and the cost savings. Let the efficiency vary from 0.6 to 0.9, and the unit cost to vary from $4 to $6 per million Btu. Plot the annual energy used and the cost savings against the efficiency for unit costs of $4, $5, and $6 per million Btu, and discuss the results.