What is the difference between a Public Branch eXchange $($PBX$)$ and Central Exchange $($CENTREX$)?$

$2$ points

a$.$ a CENTRX is owned and operated by the organization, while PBX is owned and operated by the service provider

b$.$ a PBX refers to a single organizationally owned switch that cannot be networked with other PBXs within the organization; a CENTREX represents a networked solution only offered by the service provider to interconnect PBXs

c$.$ a PBX is owned and operated by the organization, while CENTREX is owned and operated by the service provider

d$.$ PBX and CENTREX are interchangeable terms that refer only to the voice networking technology used $($i$.$e$.,$ there is no difference$)$

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