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, employer, platform, manufacturer

* 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.
product/material produced