1.) Why does menstruation begin?
2.) How long does the menstrual cycle usually last?
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1.) Why does menstruation begin?

A.) Period happens because of changes in hormones in the body. 

2.) How long does the menstrual cycle usually last?

A.) the menstrual cycle lasts 21 to 35 days while bleeding between 2 to 7 days

-TheUnknownScientist

the statements that apply here about binary star system are

"have stars that might appear to wobble"

"often have one star that is brighter than other"

a binary star is a star system consisting of two stars revolving around their common barycenter. systems of two or more stars are called multiple star systems.