I think the bicycle, since it’s cheap, environmentally friendly, and relatively quick. However, that’s just me since I ride a bike to work everyday.
The most important is probably the train, because trains made it possible to transport materials (and very large materials) across the country quite easily when things were manufactured in one place but needed to be used in another. It is something we still rely on quite a bit, even though its invention was in the early 1800’s.
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