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Among elastic oscillations of air, ultrasound (from Latin ultra — beyond, more, above), inaudible to the human ear and with a lower frequency limit conventionally taken as 20 kHz, has long...
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My house — my fortress. And the fortress must be strong lock. It is clear even to children. An example of this is Evgeny Lebedev, the pupil 3 classes of school № 3 of the city of...
