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Two astronauts working on the exterior of a space station, separated by a few metres, CANNOT hear each other's voices directly. The reason is:

AVibrations only travel in one direction
BTheir helmets are soundproof
CCosmic radiation blocks sound
DOuter space is a near-vacuum — there is no medium for sound to travel through, so voice cannot reach the other astronaut as a sound wave
Answer & Solution
Correct answer: D. Outer space is a near-vacuum — there is no medium for sound to travel through, so voice cannot reach the other astronaut as a sound wave
Sound requires a medium. The vacuum of space contains essentially no air particles to carry the vibration. So even shouting works only INSIDE the suit. Astronauts on EVA communicate by radio — electromagnetic (transverse) waves, which travel through vacuum at the speed of light.
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