At a branch location, TestKing.com has deployed a non modular router that is equipped with two serial interfaces and two Ethernet interfaces.What additional equipment must be used to allow the branch office to connect to the central office by using an ISDN service?（）
A. a serial cable with a V.35 to RJ45 adapter
B. a CSU/DSU
C. an external ISDN TA and NT1
D. The non-modular router cannot connect to an ISDN service. The network administrator must purchase a modular router with an ISDN BRI-U card.