题目选项分析

Bufo marinus toads, fierce predators that will eat frogs, lizards, and even small birds, are native to South America but were introduced into Florida during the 1930's in an attempt to control pests in the state's vast sugarcane fields.

Bufo marinus toads, fierce predators that will eat frogs, lizards, and even small birds, had been native to South America but were introduced to Florida during the 1930's as an attempt at controlling pests in the state's vast sugarcane fields.