Which two mechanisms could trigger a high availability (HA) failover event?

[Reference: PAN-OS High Availability (HA) documentation explains that ping monitoring is used to verify the path to a network resource, and failure can trigger an HA event., PAN-OS Administrator’s Guide - HA, Link monitoring:, Link monitoring checks the status of network links. If a monitored link fails, an HA failover can be triggered., Reference: Link monitoring is described in the PAN-OS documentation as a key component of the HA functionality, used to detect link failures., PAN-OS High Availability Link Monitoring, , ]