The figure shows a hand-built 'eardrum model' — a tin can with a rubber sheet stretched across one end, with cereal grains placed on the rubber.  When a person speaks loudly into the open end, the grains jump up and down because:
ASound vibrations make the air inside the can push and pull the rubber sheet, which then makes the grains jump
BThe grains are heated by the speaker's breath
CThe metal can magnetically attracts the grains
DThe grains have an internal vibration of their own
Answer & Solution
Correct answer: A. Sound vibrations make the air inside the can push and pull the rubber sheet, which then makes the grains jump
Sound waves from the voice travel through the air inside the can and exert tiny periodic pressure changes on the rubber sheet, making it vibrate like a real eardrum. The grains sitting on top get tossed up and down by these vibrations. Inside a real ear, the eardrum vibrates and the vibrations are then passed to the inner ear.
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