In Asia, where palm trees are non-native, the trees’ flowers have traditionally been pollinated by hand, which has kept palm fruit productivity unnaturally low. When weevils known to be efficient pollinators of palm flowers were introduced into Asia in 1980, palm fruit productivity increased—by up to fifty percent in some areas—but then decreased sharply in 1984.
Which of the following statements, if tree, would best explain the 1984 decrease in productivity ？（）
A．Prices for palm fruit fell between 1980 and 1984 following the rise in production and a concurrent fall in demand.
B．Imported trees are often more productive than native trees because the imported ones have left behind their pests and diseases in their native lands.
C．Rapid increases in productivity tend to deplete trees of nutrients needed for the development of the fruit-producing female flowers.
D．The weevil population in Asia remained at approximately the same level between 1980 and 1984.
E．Prior to 1980 another species of insect pollinated the Asian palm trees, but not as efficiently as the species of weevil that was introduced in 1980.