[单选题]45、 Which of the following is not an available method of HA for use with VPN connectivity?当和VPN连接一起使用时，下面的哪种方法不是适用的HA方法?
A. Active/standby failover active/standby故障切换
B. Active/active failover active/active故障切换
C. VPN load balancing VPN负载平衡
D. External load balancer 外部负载平衡器