What is the relationship between energy company and natural gas 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.

* 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