What factor contributes to cities transitioning from outward expansion to upward expansion?

• A. Electricity
• B. Cement
• C. Steel
• D. Wood

Answer: (C)

The transition of cities from outward expansion, where urban areas spread horizontally, to upward expansion, where they build vertically, is heavily influenced by the availability and use of steel. Steel is a strong and durable material that allowed for the construction of taller buildings, such as skyscrapers, which became feasible due to its strength-to-weight ratio. This advancement in building technology enabled cities to utilize limited land more efficiently by constructing multi-story structures, rather than continuing to spread out across the landscape.

Electricity, while critical for urban development and enhancing the functionality of buildings, does not directly contribute in the same way to the structural capability needed for upward growth. Cement is important for building foundations and as a component of concrete, but it lacks the tensile strength required for constructing very tall buildings. Wood, on the other hand, is more suited for low-rise structures and does not provide the necessary support for high-rise buildings, especially in urban areas where space is at a premium.

Overall, steel's role in enabling the construction of skyscrapers directly supports the shift towards upward urban expansion, making it the key factor in this transformative process.