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A sound wave's frequency is 256 Hz (middle C) in air at 340 m/s. In hydrogen gas where speed is 1300 m/s, the FREQUENCY:

ADecreases
BIncreases
CDisappears
DStays the same (frequency is set by the source)
Answer & Solution
Correct answer: D. Stays the same (frequency is set by the source)
Frequency is determined by the SOURCE (the vibrating object), not the medium. In hydrogen, the same 256 Hz frequency persists; the WAVELENGTH changes (since v = λν, larger v means larger λ at fixed ν). So in hydrogen, λ = 1300/256 ≈ 5.08 m (much longer than in air's 1.33 m).
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