ComforTech management learns that last month's production run of space heaters at its Iowa plant were found to have excessive defects in quality control tests. The shut-off switch does not work properly, which is a major fire hazard. In response, the firm announces a voluntary recall of the heater. Having sold a large number through retailers Lowe's and Home Depot, ComforTech expects a large number to return. Consumers will return heaters to those retailers who will then ship units back to ComforTech, which means the units will travel through a __________ distribution channel