Fact 1: Electricity is used as an energy source by electrical devices.

Fact 2: Devices such as pagers and cell phones are forbidden.

Given the two facts above, answer the question "What is used as an energy source by pagers?" with the following options: 
- Wi-Fi
 - photons
 - antennae
 - energy
 - Heart rate
 - fire
 - layers of fat
 - electricity


 

        electricity 