To reach fifty million users, it took
a) radio four years and tv thirteen years
b) the internet forty years and tv thirteen years
c) about the same time as it took radio and tv
d) the internet one tenth (1/10) of the time it took radio

I got the Internet one tenth (1/10) of the time it took radio

Explanation:

"75 years for the telephone to connect 50 million users. The radio hit this goal in 38. It wasn't until 13 years till 50 million people were sitting in front of their television screens in their homes. The Internet in four years"