You have just obtained a modular Cisco router with 2 serial connections and a BRI/U interface. What other physical piece of hardware is required in order to establish an ISDN connection?（）

• A、Nothing. The router is already suitable.
• B、A BRI WAN interface in the router.
• C、An external NT1 to terminate the local loop.
• D、A TA/NT1 device on the router.