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