单项选择题 Which protocol is an open standard protocol framework that is commonly used in VPNs,to provid secure end-to-end communication?（）

单项选择题 which command is used to enable CHAP authentication whit PAP as the fallback method on a serial interface?（）

单项选择题 Refer to the exhibit.Statements A,B,C,and D of ACL 10 have been entered in the shown order and appled to interface E0 inbound,to prevent all hosts(except those whose addresses are the first and last IP of subnet 172.21.1.128/28) from accessing the network.But as is,the ACL dose not restrict anyone from the network.How can the ACL statements be re-arranged so that the system works as intended?（）