When the market price of copper piping increased significantly, housing developers began using plastic (PEX) piping instead of copper in new plumbing installations. This indicates that plastic (PEX) piping has the following relationship with copper piping:
plastic (PEX) piping is a complement to copper piping
plastic (PEX) piping is a substitute for copper piping
plastic (PEX) piping is an inferior good
plastic (PEX) piping is a free good
