The water of Lake Laberge, in Canada, currently contains high levels of the pesticide toxaphene. Authorities are puzzled because toxaphene was banned in North America in the early 1980s and now is used only in a few other parts of the world. Which one of the following, if true, does most to explain why the water of Lake Laberge currently contains high levels of toxaphene? ['North American manufacturers opposed banning toxaphene.', 'Levels of pesticides in the environment often continue to be high for decades after their use ends.', 'Toxic chemicals usually do not travel large distances in the atmosphere.', 'Toxic chemicals become more readily detectable once they enter organisms the size of fish.']