Fact 1: Atelephone is used for communication by humans over long distances.

Fact 2: Long-distance telephone calls can also use radio waves or microwaves.

Given the two facts above, answer the question "What waves does a telephone use?" with the following options: 
- unique
 - power
 - energy
 - fibers
 - sound
 - strong
 - Radio
 - two


 

       Radio  