A local environmental group has undertaken a multi-million dollar project that has successfully removed almost all of the litter, trash and debris from the Allegheny river.For every additional mile of river they clean, ecotourism increases by $1,000.The cost for cleaning the first additional mile is $750, the second is $995, the third is $1,200 and the fourth is $1,500.They have asked for your advice on whether they should proceed to clean the next four miles or if they should stop their efforts.Usemarginal analysisto help them decide what would be the besteconomicaldecision.