What does 3PCC mean?

What does 3PCC mean?
A . All call handling actions (answer, hold, initiate transfer, etc.) are performed on the SIP endpoint
B . All call handling actions (answer, hold, initiate transfer, etc.) are performed on the multiple PCs (In this case 3 PCs)
C . T-lib signaling takes over SIP signaling, so third party application such as Agent Desktop can transfer VoIP packets to SIP Server and back using T-lib
D . A T-lib client such as Agent Desktop can control a call which is actually between other parties such as SIP endpoints

Answer: D