Two Technicians Are Discussing Pulse Train Signals

Two technicians are discussing pulse train signals. Technician A says that a pulse train is a DC signal waveform that goes above and below zero volts. Technician B says that a pulse train is a DC voltage that turns on and off in a series of pulses and they do not go below zero. Which technician is correct?

A) Technician A only
B) Technician B only
C) Both technicians A and B
D) Neither technician A nor B