The correct answer is D). Availability of cheaper iron.
The skyscraper depended on all the following technological innovations except the Availability of cheaper iron.
In the beginning, the term Skyscraper is from the 1880s. It was used to name tall buildings of that time with 10-20 floors. Nowadays, a Skyscraper is for buildings 40-50 floors tall.
Generally, Skyscrapers are built with fireproof steel frames of cast iron columns and beams. The use of an elevator was very important for the success of this new type of buildings.
