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(1) skyscrapers were made possible by two innovations in technology: structural steel (steel shaped for use in construction) and the elevator. (2) previously, buildings were supported by their walls. (3) large buildings had to be built with walls of heavy masonry (stonework or brickwork). (4) masonry walls made it difficult to put up structures higher than a hundred feet because the upper walls would weigh too much for the lower stories to bear. (5) steel changed all that completely. (6) the weight of the building could now be borne entirely by a light steel skeleton frame. (7) the outside walls, completely relieved from the duty of supporting weight, were now a mere “skin” and could be made of anything: tiles, metal panels, and glass. (8) and with the elevator, buildings could climb until they scraped the sky.

1. in sentence 1, the word innovations means

a. inventors.

b. new things.

c. buildings.

d. questions.