Fact 1: Telephones are used for communication by two people by transmitting sound.

Fact 2: Loudspeakers then turn the signals into sounds.

Given the two facts above, answer the question "What are used for communication by two people by transmitting signals?" with the following options: 
- electricity
 - satellites
 - air
 - phytoplanktons
 - unidirectional
 - telephones
 - the Sun
 - waves


 

      telephones   