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 very   frequent 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