Pairwise beat frequencies are |400-401|=1 Hz, |401-402|=1 Hz and |400-402|=2 Hz, so audible beat components include 1 Hz and 2 Hz, effectively two distinct beat rates; option C is appropriate.

Lighter gases (lower molecular mass) have higher speeds of sound, so hydrogen yields the maximum speed under similar conditions.

For a closed pipe f1 = v/(4L)=334.2/(4*0.6)=139.25 Hz; the third overtone is the 7th harmonic: 7*f1 ≈ 7*139.25 = 974.75 Hz.

By the Doppler effect, when source and observer move apart the observed frequency (pitch) decreases.

Intensity ∝ amplitude^2 × frequency^2 (via particle velocity ∝ ωA), so factor = (2^2)×(1/4)^2 = 4×1/16 = 1/4, a decrease by factor 4.

Speed of sound is affected by temperature, humidity and medium density, so 'All of these' is correct.

For a string at same tension f ∝ 1/L, so f2 = 250*(0.60/0.40) = 250*1.5 = 375 Hz.

Prolonged exposure to sound levels around 130 dB can cause hearing damage; this is the typical threshold for potential hearing injury.

For a pipe closed at one end, fundamental wavelength λ = 4L = 100 cm for L = 25 cm; the OCR shows option C marked.

1 bel corresponds to a factor of 10 in intensity, so 2 bel corresponds to 10^2 = 100 times greater intensity.