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.