What is the relationship between rapper and music streaming in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

In 2013 he joined rapper Dr Dre's headphones and music streaming service Beats Music, which was bought by Apple for $3bn in May last year .

Answer: distribution_format