A、It created design problems for architects. B、It was needed for transporting construction materials. C、It enabled architects to

(20)In 1871 the first passenger elevators were used in office buildings and allowed architects to build higher than people could comfortably walk. Another innovation was in building technique.(21)In 1885 the steel skeleton was introduced and allowed for the construction of tall buildings that could withstand high winds. We take for granted some of the other inventions that enabled people to live and work in skyscrapers. For example, few people realized that the telephone was necessary for vertical communication and that flush toilet and vacuum incinerator made waste disposal possible. Now as we entered the age of super skyscraper, some with more than 200 floors, we see the need for even more technological innovations. In the area of heating and cooling systems for example, for all their benefits, these super tall buildings do cause problems though. For one thing they place enormous strain on parking and traffic row in urban areas.(22)But let’s leave behind these technical concerns and move on to consider some of the design elements that have come to characterize the age of the skyscraper.
20. What does the professor mainly discuss?
21. What does the professor say about the invention of the elevator?
22. What will the professor probably discuss next?