Question: In inter-operator charging for resource utilization in supporting user sessions, what is a possible difference in charging methods between operator-to-operator and operator-to-user billing?
Option 1: Currency used for invoicing (e.g., USD for users, EUR for operators)
Option 2: Metric of consumption measured (e.g., duration for users, volume of data for operators)
Option 3: Frequency of billing cycles (e.g., monthly for users, quarterly for operators)
Option 4: Type of services billed (e.g., voice for users, data and voice for operators)
Explanation: This is correct because, as highlighted in the context, the charging method can differ between operator-to-operator and operator-to-user. Specifically, a user might be charged based on the **duration** of a voice session (e.g., per minute), whereas the mobile network operator might pay a fixed network or 3rd party carrier based on the **volume of data used** (e.g., per GB). This illustrates a difference in the metric of consumption measured for billing purposes between the two types of billing relationships.