In what situation would the command ip helper-address be required？（Choose the best answer.）（）

• A、Only when there is a duplicate IP address caused by a combination of static and dynamic IPaddress allocations
• B、On each router that exists between the client and the server
• C、Only when a router separates the client from the server
• D、Only if the DHCP sever issues a DHCPNAK to the initial request
• E、Only when the client is on the same subnet as the server
• F、Only when the DHCP pool is out of IP addresses