Several years ago, as a measure to reduce the population of gypsy moths, which depend on oak leaves for food, entomologists introduced into many oak forests a species of fungus that is poisonous to gypsy moth caterpillars. Since then, the population of both caterpillars and adult moths has significantly declined in those areas. Entomologists have concluded that the decline is attributable to the presence of the poisonous fungus. Which one of the following, if true, most strongly supports the conclusion drawn by the entomologists? ['The current decline in the gypsy moth population in forests where the fungus was introduced is no greater than a decline that occurred concurrently in other forests.', 'In the past several years, air pollution and acid rain have been responsible for a substantial decline in oak tree populations.', 'A strain of gypsy moth whose caterpillars are unaffected by the fungus has increased its share of the total gypsy moth population.', 'An increase in numbers of both gypsy moth caterpillars and gypsy moth adults followed a drop in the number of some of the species that prey on the moths.']