How do modems (modulation/demodulation) function to facilitate analog transmissions to enter a digital network?

A. Modems convert analog transmissions to digital, and digital transmission to analog.
B. Modems encapsulate analog transmissions within digital, and digital transmissions within analog.
C. Modems convert digital transmissions to analog, and analog transmissions to digital.
D. Modems encapsulate digital transmissions within analog, and analog transmissions within digital.