Louis sullivan used which type of construction in the late 19th century in chicago to build increasingly tall buildings?

Louis Sullivan, an American architect, nicknamed the "father of skyscrapers" was known for his famous construction of very tall buildings in the late 19th century. Curiosity rises as to which type of construction he has been using. Reinforced concrete of steel was his key way to make increasingly tall buildings.

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The answer is option a. boundary line drawn by pope alexander iv in 1493.