A. opportunity cost
B. scale of preference
C. complementary demand
D. double of coincidence of wants
Correct Answer: Option D
D. double of coincidence of wants
Explanation
This occurs when two people have goods they are both happy to swap in exchange. i.e. a perfect barter exchange. a requirement that must be met before a trade can be made. It specifies that a trader must find another trader who is willing to trade what the first trader wants and at the same time wants what the first trader has.