Utilize the input text as a context reference, choose the right relationship between rapper and music streaming from the options. Options: 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