【单选题】
What is the difference between a CSU/DSU and a modem?（）
A、A CSU/DSU converts analog signals from a router to a leased line; a modem converts analog signals from a router to a leased line.
B、A CSU/DSU converts analog signals from a router to a phone line; a modem converts digital signals from a router to a leased line.
C、A CSU/DSU converts digital signals from a router to a phone line; a modem converts analog signals from a router to a phone line.
D、A CSU/DSU converts digital signals from a router to a leased line; a modem converts digital signals from a router to a phone line.
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