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The answer to this question is B. Elevators
One of the reason why skyscrapers were not popular before the invention of elevators was that it took too much energy to move from the groud floor to the upper floor with the stairs alone.
After the invention of elevators, we eliminate this problem and people could move from ground floor to upper floor with only little effort.
I think its the word world theory but idk I could be wrong I'm just picking a random word that I try to fit in the sentence!
