题目选项分析

The gypsy moth, which in its caterpillar stage poses a great threat to fruit crops in the state of California, has been subjected to regular spraying of several pesticides by the USDA. These pesticides, while not uniformly successful, have been shown to be effective in killing a broad range of insects in the adult stage. Research has also been done on the effect of dusty visalia beetle on gypsy moth populations. In lab tests, this natural enemy of the gypsy moth successfully controlled gypsy moth population growth by feeding on the cocoons of the moth without harming the fruit trees. Yet, when the USDA released millions of these beetles in the state's orchards, they had little effect.

Which one of the following, if true, would offer the most likely explanation for the failure of the USDA's dusty visalia beetle-release program?

【选项】The cost of growing the fruit trees used in the study far exceeded the cost of raising the dusty visalia beetle.