Passage Summary: We've saved a lot of money by making buildings more energy-efficient since the 1970s. Because we are currently saving billions every year, the author predicts those savings will skyrocket to over $200 billion annually in the future.

Conclusion: In 50 to 100 years, energy-efficient building technologies will save more than $200 billion annually in today's dollars.

Reasoning: Building energy use exceeds transportation energy use, and current efficiency technologies already save several billion dollars per year.

Analysis: The argument relies on a massive numerical leap from 'several billion' today to 'over $200 billion' in the future. To make this prediction valid, the author must assume that the factors driving these savings—such as the total amount of energy used or the cost of that energy—will remain significant or grow. If energy were to become virtually free or if buildings stopped using energy altogether, those savings would vanish. Look for an answer that ensures the scale of energy consumption or its cost remains high enough to make such massive savings possible.