What is a key feature of A.C. current?

A key feature of alternating current (A.C.) is that it moves to and fro, meaning that the direction of the flow of electricity periodically reverses. This characteristic is what distinguishes A.C. from direct current (D.C.), where the flow of electric charge consistently moves in one direction. In A.C. systems, the voltage and current change polarity with a specific frequency, typically measured in Hertz (Hz), indicating how many times the flow reverses direction in one second. This to-and-fro movement allows for more efficient transmission of electricity over long distances, as it can easily be transformed to higher or lower voltages.

In contrast, the other options describe properties that do not apply to A.C. current, underscoring the correctness of the chosen answer.