Take context from the input text and decide on the accurate relationship between 'energy company' and 'natural gas' from the options provided.
Options: employer, platform, manufacturer, product/material produced

* 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