Fact 1: Electric devices require electrical energy to function.

Fact 2: Ordinary phone is also a mobile device.

Given the two facts above, answer the question "What do phones require to function?" with the following options: 
- encoded information
 - graduated cylinderr
 - math
 - electrically
 - electrical energy
 - magnifying glass
 - hydrogen and oxygen
 - electromagnetic energy


 

     electrical energy    