The best description of an operational level Agreement（OLA）is( )。
A． an agreement between the serivice provider and another part of the same Drganization
B．a(chǎn)n agreement between the serivice provider and an exterral organization
C．a(chǎn) docament that describes how services will be operated on a day -to -day basis to customer
D．a(chǎn) docament that business services to operational staff