单项选择题
A.DLCI 17 describes the ISDN circuit between R2 and R3.
B.DLCI17 describes a PVC on R2.It cannot be used on R3 or R1.
C.DLCI17 is the Layer 2 address used by R2 to describe a PVC to R3.
D.DLCI17 describes the dial-up circuit from R2 and R3 to the service provider.
