What distinguishes alternating current (AC) from direct current (DC)?

Alternating current (AC) is characterized by its sinusoidal waveform, which means that the current changes direction periodically and the voltage varies in amplitude, typically in a smooth, wave-like pattern. This alternating nature allows AC to easily change voltage levels using transformers, making it highly suitable for long-distance transmission of electricity.

In contrast, direct current (DC) flows in a constant direction and maintains a constant voltage, displaying a steady level of current over time. This unidirectional flow is commonly supplied by batteries and is ideal for applications like electronic circuits where stable voltage is essential.

Understanding the waveform is crucial as it directly influences how AC and DC are utilized in various applications. The sinusoidal waveform of AC allows for efficient transmission and distribution, while DC's steady current is essential in powering devices that require a constant voltage. Hence, option C effectively captures the fundamental difference between AC and DC.