Which of the Following Definitions Best Describes Direct Current

Which of the following definitions best describes direct current?

A) The unidirectional uninterrupted flow of charged particles with a duration of at least one second
B) The bidirectional uninterrupted flow of charged particles changing direction at least once per second
C) The unidirectional or bidirectional flow of charged particles periodically ceasing for less than one second before the next electrical event
D) A monophasic pulse that changes phase at least twice within one second