To send digital information, we must devise analog signals to represent bits. The process of converting between bits and signals that represent them is called digital modulation. With digital modulation:

A.the most straightforward form of digital modulation is to use a positive voltage to represent a 1 and a negative voltage to represent a 0, a scheme called NRZ (Non-Return-to-Zero)

B.schemes that directly convert bits into signals (occupying frequencies from 0 up to a maximum) result in passband transmission

C.it is possible to share a channel by multiple signals, this scheme is called baseband transmission

D.all of the above are true