Audible Range: 25 Hz to 20 kHz Steps:

• The audible frequency range for a healthy human ear is 25 Hz to 20 kHz.
• Frequencies below 25 Hz (infrasound) and above 20 kHz (ultrasound) are inaudible.
• Examine each option to identify the pair where both frequencies fall outside this range.
• Option B contains 20 Hz (below 25 Hz) and 25 kHz (above 20 kHz), satisfying the condition.

Why B is correct:

• 20 Hz is below the 25 Hz lower limit, and 25 kHz exceeds the 20 kHz upper limit, per the standard human hearing range definition.

Why the others are wrong:

• A: 500 Hz is within 25 Hz to 20 kHz.
• C: 25 Hz is at the lower limit (audible).
• D: 500 Hz is within 25 Hz to 20 kHz.

Final answer: B