Passage Summary: Steel plants throw away a ton of heat. If they used special devices to turn that heat into power, they'd pay less for electricity and save money.

Conclusion: Steel-manufacturing plants could save money by using thermophotovoltaic generators to turn waste heat into electricity.

Reasoning: Steel manufacturing creates a lot of waste heat, and these generators can convert that heat into electricity, which would lower the plants' electric bills.

Analysis: The engineer is assuming a net financial gain, but they only mention the reduction in the electric bill. The 'gap' here is the cost of the generators themselves. For this to be a necessary assumption, the argument must believe that the cost of buying, installing, and running these generators isn't higher than the money saved on electricity. Look for an answer that ensures the 'savings' aren't wiped out by hidden expenses.