Telephones are used for communication by two people by transmitting sound.
Sound sound waves are mechanical waves.
Telephones are used for communication by two people by transmitting mechanical waves. .
Telephones are used for communication by two people by transmitting what? 
[1] Transmitting illness.
[2] chemical messengers
[3] encoded information
[4] electromagnetic energy
[5] Television.
[6] Communication degree.
[7] Mechanical waves.
[8] Electrical energy