Study the exhibit carefully. Routers A and B are customer routers. Routers 1， 2， 3 and 4 are provider routers. The routers areoperating with various IOS versions. Which frame mode MPLS configuration statement is true？（）

• A、Before MPLS is enabled， the ip cef command is only requited on routers 1 and 4
• B、After MPLS is enabled， the ip cef command is only required on routers 1 and 4
• C、Before MPLS is enabled， the ip cef command is only required on the Ethernet 0 interfaces ofrouters 1 and 4
• D、After MPLS is enabled， the ip cef command is only required on the Ethernet 0 interfaces ofrouters 1 and 4
• E、Before MPLS is enabled， the ip cef command must be applied to all provider routers