How Does an IGMP Router Maintain Group Memberships on a Network

How does an IGMP router maintain group memberships on a network? (Select 2 answers)

A) The router maintains a timer for each group membership which it refreshes for hosts that send report messages and deletes the group membership of hosts that do not.
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.