What is the difference between a PBX and a keysystem under the Device tab?

You are CCNA VOICE associate in Certkey.com. What is the difference between a PBX and a keysystem under the Device tab?

A.
each phone has nearly identical configuration in PBX but a keysystem has a unique extension for each phone.

B.
each phone has a unique extension in PBX but a keysystem has nearly identical configurations for each phone.

C.
A PBX operates 24×7 and requires no activation.

D.
A keysystem can be activated only by the operator by turning the Systemkey.