When troubleshooting a T1 connection, the support representative from the provider instructs a technician to
place a special device into the CSU/DSU. Using this device the provider is able to verify that communications
are reaching the CSU/DSU. Which of the following was MOST likely used by the technician?

A.
Cable analyzer

B.
Toner probe

C.
OTDR

D.
Loopback plug