At various times, the nations that constitute the Organization of Petroleum Exporting Countries (OPEC) have restricted the supply of oil to increase their profits. This is an example of:

a the unsuitability of some goods for efficient management by markets.
b regulating self-interest.
c individual actions whose side effects are not properly taken into account by the market.
d one party preventing mutually beneficial trades in an attempt to capture a greater share of resources for itself.