Question

in one complete paragraph, identify the single most significant turn-of-the-century invention, and explain why you think it is so important to life today

Answer 1

I would say the Internet. The Internet has helped to connect people digitally around the world. It provides quick sources of entertainment and information.
If we want to know what's airing at the nearest cinema we can just look it up. If we want letters to be sent quickly that can happen in a second. 

Answer 2

This depends on the century you are referring to. If you mean the transition from the 1800s-1900s, then perhaps the most important invention was the telephone, the locomotive or electricity. All of these inventions transformed the way we communicated with one another, lived our daily life, and travelled.

If what you mean is the transition from the 1900s to the 2000s, then the most important invention is most likely the Internet. The Internet has allowed us to unite the world in a way that has never been seen before. Information can travel faster than ever, communication has been revolutionized, transactions have become extremely simple and our world has become digital.