[单选题]13、 Which of the following HA methods are not supported by clientless VPN connections?无客户端的VPN连接不支持下面哪一个HA方法?
A. Redundant peering 冗余对等体
B. Clustering 集群
C. Active/standby failover active/standby故障切换
D. Load balancing using an external ACE 使用外部ACE的负载平衡