How Does an IGMP Router Maintain Group Memberships on the Network

How does an IGMP router maintain group memberships on the network? (Choose two.)

A) The router maintains a timer for each group membership. If the router receives a Report message from a receiver host, it refreshes the timer of the group membership of this host; if the router does not receive any Report message from a receiver host before the timer expires, it deletes the group membership of this host.
B) The router periodically sends Query messages. After a receiver host receives the Query message, the receiver host sends a Report message.
C) A receiver host periodically sends a Report message.
D) The router deletes a group membership only when it receives a Leave message for this group.