DC here means "direct current". Direct current is characterized by it's flow of current been <u>unidirectional</u> (meaning it's current flows in just <u>one direction</u>). Thus, to get the answer to the question one must look for options that only move in a single direction. Options A and B move in a back and forth direction meaning they are not unidirectional.
A longitudinal wave moves in the same direction OR opposite direction to the direction of the propagating wave. This means longitudinal wave is also unidirectional. <u>Option C is correct</u>.
1st graders, in a line, going to the cafeteria also move in one direction (direction of the cafeteria). Thus, <u>option D is also correct.</u>
