Which combination of factors contributed to the invention of the skyscraper?

• A. Introduction of air conditioning
• B. Invention of the passenger elevator and steel skeletons; urban density demands
• C. Use of reinforced concrete
• D. Development of balloon framing

Answer: (B)

Tall buildings could rise only when two practical advances lined up with city needs. A steel skeleton lets a structure bear its weight with a lightweight external frame, so designers can make tall forms with thinner walls instead of thick load-bearing supports. At the same time, a reliable passenger elevator makes moving people quickly between many floors feasible, turning height from a novelty into a workable office or residence solution. When cities became crowded and land was costly, going up offered a way to increase usable space without widening the footprint, fueling the push for skyscrapers. The other factors—air conditioning, reinforced concrete, or balloon framing—played important roles in related areas of construction, but they don’t address the essential combination that first made tall, vertical urban architecture practical and desirable.