Basic General Knowledge
To produce sound it is necessary that

the vibrations of source are not necessary
the source should execute transverse vibrations
the source may execute any type of vibrations
the source should execute longitudinal vibrations

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Basic General Knowledge
Primary rainbow is formed when light suffers

either one or two internal refractions before emerging out of the drop
no internal refraction
two internal refractions before emerging out of the drop
one internal refractions before emerging out of the drop

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