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. and Patriot Coal Corp. are among the miners that filed for Chapter 11 this year due to unsustainable debt loads and interest expenses.

Answer: product_or_material_produced