Why should a magnesium ribbon be cleaned before burning in air?

Why should a magnesium ribbon be chleaned before burning in air ?

Why should a magnesium ribbon be cleaned before is burnt in air ?

With what, magnesium ribbon is cleaned before buning it in air.

Assertion (A) : Magnesium ribbon is cleaned with water before burning . Reason (R ) : MgO layer is formed on magnesium ribbon , on exposure to atmosphere .

A burning magnesium ribbon will continue to burn in

A burning magnesium ribbon will continue to burn in

Carbondioxide does not support combustion but a burning magnesium ribbon continues to burn in it. Why?