Question: In MBMS (Multimedia Broadcast/Multicast Service), what is the primary purpose of including a `r12:unicastAppService` element with a `basePattern` entry that matches a broadcast resource's URL?
Option 1: To prioritize unicast delivery over broadcast for enhanced user experience.
Option 2: To enable the MBMS client to provide unicast fallback when broadcast resources are unavailable, ensuring service continuity.
Option 3: To mandate the use of externally defined policies for resource allocation in MBMS services.
Option 4: To restrict access to certain resources by only offering them through unicast delivery.
Explanation: The `r12:unicastAppService` element with a matching `basePattern` allows the MBMS client to identify resources available via unicast that are redundant to broadcast resources, enabling uninterrupted service continuity when broadcast resources cannot be accessed.