Routers R1 and R2 are configured for HSRP as shown below： Router R1： interface ethernet 0 ip address 20.6.2.1 255.255.255.0 standby 35 ip 20.6.2.21 standby 35 priority 100 interface ethernet 1 ip address 20.6.1.1.2 255.255.255.0 standby 34 ip 20.6.1.21 Router R2： interface ethernet 0 ip address 20.6.2.2 255.255.255.0 standby 35 ip 20.6.2.21 interface ethernet 1 ip address 20.6.1.1.1 255.255.255.0 standby 34 ip 20.6.1.21 standby 34 priority 100 You have configured the routers R1 & R2 with HSRP. While debugging router R2 you notice veryfrequent HSRP group state transitions. What is the most likely cause of this？（）

• A、physical layer issues
• B、no spanning tree loops
• C、use of non-default HSRP timers
• D、failure to set the command standby 35 preempt