Utilize the input text as a context reference, choose the right relationship between energy company and natural gas 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.

* Black Hills (BKH.N) rose 1.3 percent to $124.88 a day after the energy company said it would buy natural gas utility company SourceGas Holdings for about $1.9 billion.

Answer: product_or_material_produced