In terms of mercantilism, what is protectionism?
A. a political policy that states that a nation should take over foreign ports by force if necessary to insure healthy trading relations
B. an economic policy in which a nation spends a great deal of money on its navy in order to defend it merchant ships
C. a trade policy that involves undercutting the prices of other an nation's goods at all costs
D. a trade policy the encourages exports while discouraging imports, mainly through tariffs