Which statement best distinguishes alternating current from direct current?

• A. AC reverses direction periodically; DC flows in a single direction
• B. AC flows in a single direction; DC reverses
• C. AC has no voltage; DC has voltage
• D. AC is always safe; DC is not

Answer: (A)

The key difference is how current behaves over time: alternating current reverses direction periodically, so the current and voltage polarity swap back and forth, typically in a waveform like a sine wave. Direct current flows in a single direction from the source and stays that way while the source is active. Because of this, AC voltage varies with time and can cross zero and change sign, whereas DC voltage remains relatively constant (though real systems can have small ripple). The statement that AC has no voltage is incorrect because AC involves a voltage that changes with time. The idea that AC is always safe is not a defining trait of the current type—safety depends on voltage level, insulation, and other factors, not whether the current is AC or DC. The notion that AC flows in a single direction and DC reverses is the opposite of how these currents actually behave.