What aspect of human nature, that still exists today, was revealed when a self-propelled steam engine was first put to use in the U.S. in 1841?

The question points to resistance to a new invention from people who feared it would threaten their jobs. Firefighters sabotaged the self-propelled steam engine because they worried it would make their work unnecessary. This reveals that people may oppose technological change when they fear losing their livelihood; choice D states this directly.

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