Question

What are two inventions that made modern city life possible?

I know one of the inventions are skyscrapers but I am not certain of the answer to the other one.
Ms. Sue
Plumbing with sewage treatment plants is even more important than skyscrapers.

Also -- if you have skyscrapers, you must also have Otis's invention.

http://inventors.about.com/od/estartinventions/a/Elevator.htm