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.