Concept:Double coincidence of wants is the need for two traders in barter to each hold what the other desires.Explanation:Under a barter system, a sale can succeed only when a trader with surplus goods finds another person who both wants those goods and can offer in return the goods the first trader needs.Matching these mutual wants is very difficult in practice.This difficulty is precisely the problem of double coincidence of wants.Answer:D. one has to find someone who wants his surplus and also has what he needs