In Non Return to Zero Encoding Schemes,the Signal or Voltage

In Non Return to Zero encoding schemes,the signal or voltage level does not return to 0v (zero volts) .One of the problems with this type of encoding is that

A) it does not work well with serial communications.
B) it is designed primarily for high speed communications only.
C) the signal ground of the transmitter is often different from that of the receiver
D) it works only for analog signals.
E) it is sometimes difficult to synchronize the timing of the transmitter and the receiver.