Which kind of listener supports this feature?

To meet business requirements, a user requires that the backend server of an SLB instance include a primary ECS instance and a secondary ECS instance. When the primary instance is working properly, the traffic is only forwarded to the primary ECS, when the primary instance is down, traffic is switched to the standby instance. In this case, you can create an active/standby server group on the backend of the SLB instance.

Which kind of listener supports this feature? (Number of correct answers: 2)
A . HTTPS
B . HTTP
C . UDP
D . TCP

Answer: A,C