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 addresse