The table below describes four atoms. 
Which atoms represent the same element?
Definitions:
Barrier
An obstacle that prevents movement or access.
Disclaimer
A statement of denial of legal liability or that refutes the authenticity of a claim.
Tracking System
A system designed to monitor and record the movement or progress of items, information, or people.
Online Help
Assistance provided over the internet, often through websites, chat support, or online documentation, to solve problems or answer queries.
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