Which of the following is true? a. voluntary trades give both parties more in value than what they give up. b. people can gain by specializing in the production of the good in which they have a comparative advantage. c. all of the above are true. d. without the ability to trade, people would not tend to specialize in those areas where they had a comparative advantage. e. voluntary trade directly increases wealth by making both parties better off, and it is the prospect of wealth-increasing exchange that leads to productive specialization.