What do main idea mean of the passage

The world trade organization (wto), which was formed in 1995, trades without discrimination. which of these best describes what this principle means? all its member countries charge the same prices for basic commodities such as wheat and lumber. b) all its member countries do not trade with countries which had discriminatory racial policies. c) all its member countries trade only with countries that sign the wto agreement. d) all its member countries open their markets equally to all member nations. answer is d