You are presented with the question "What are used for communication by two people by transmitting signals?" and the following answer choices: 
- electricity
 - satellites
 - air
 - phytoplanktons
 - unidirectional
 - telephones
 - the Sun
 - waves

Now knowing that telephones are used for communication by two people by transmitting sound and loudspeakers then turn the signals into sounds, choose the best answer.


 

      telephones   