Answer:
The correct options here are C and D
Step-by-step explanation:
DC here means "direct current". Direct current is characterized by it's flow of current been unidirectional (meaning it's current flows in just one direction). 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. Option C is correct.
1st graders, in a line, going to the cafeteria also move in one direction (direction of the cafeteria). Thus, option D is also correct.