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
What is a compound microscope?

A microscope that has two sets of lenses : an occular lens and an eyepiece.
A microscope that has one lens.
A microscope whose lenses are convex.
A microscope whose lenses are concave.

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