An agent In Avaya Aura Contact Center steps away from the phone set without logging out or going into Not Ready- Upon re

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An agent In Avaya Aura Contact Center steps away from the phone set without logging out or going into Not Ready- Upon return. It is discovered that the telephone has been logged out.
What is the most likely explanation?

A. The "Longest Idle since Last ACD/CDN" calls expired, logging out the telephone.
B. The Call Presentation Class "Return To Queue" was enabled for this telephone, a call was presented and returned to the queue while the agent was away and the Logout option was assigned for After "Return to Queue, Make Phone set".
C. The Call Presentation Class "After Call Break for N seconds" was enabled for this telephone, and the telephone was forced logged out.
D. The "Longest Idle since Login" expired, logging out the telephone.