What is the relationship between Walter Energy and Coal 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.

Alpha Natural Resources Inc., Walter Energy Inc., Patriot Coal Corp. and James River Coal Co. have filed for bankruptcy during the past 16 months after the price of coal used in steelmaking dropped 67 percent since 2011.

Answer: product_or_material_produced