At which layers of the OSI model do WANs operate (Choose two.)（）。

A.A DHCP client uses a ping to detect address conflicts.
B.A DHCP server uses a gratuitous ARP to detect DHCP clients.
C.A DHCP client uses a gratuitous ARP to detect a DHCP server.
D.If an address conflict is detected, the address is removed from the pool and an administrator must resolve the conflict.
E.If an address conflict is detected, the address removed from the pool for an amount of time configurable by the administrator.
F.If an address conflict is detected, the address is removed from the pool and will not be reused until server is rebooted.

A.DLCI 17描述了R2和R3之间的ISDN线路
B.DLCI 17描述了一条在R2上的PVC 不能用在R3和R1上
C.DLCI 17是R2使用的二层地址，用来描述一条到R3的PVC
D.DlDLCI 17描述了一条从R2 R3到服务提供商拨号网络

A.It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
B.It enables equipment from different vendors to use the same electronic components, thus saving research and development funds.
C.It supports theevolution of multiple competing standards, and thus provides business opportunities for equipment manufacturers.
D.It encourages industry standardization by defining what functions occur at each layer of the model.
E.It provides a means by which changes in functionality in one layer require changes in other layers.