Question

the skyscraper depended on all the following technological innovations except a the elevator. b fireproofing. c the internal steel frame. d availability of cheaper iron.

Explanation:

Skyscrapers rely on elevators (for vertical transport), fireproofing (for safety), and internal steel frames (for structural support). Cheaper iron is not a key innovation for skyscrapers; steel (not just cheaper iron) and specific engineering innovations are more relevant.

Answer:

D. availability of cheaper iron.