Physics
When two similar sources vibrate with slightly different frequencies, a regular rise and fall occurs in the loudness of tone. This phenomenon is known as

Echo
Resonance
Beats
Doppler effect

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Physics
The wall of a dam is broader at the base

To withstand pressure that is increased with atmospheric pressure
To withstand pressure that increases in a horizontal plane
Because streamlining is required
To withstand pressure that increases with depth

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