Few insects have inspired as much fear and hatred as the diminutive fire ants, less than half an inch long but living in colonies of more than 250,000 others. Everyone in the southern United States gets to know fire ants sooner or later by painful experience. Fire ants live in large earthen mounds and are true social insects--that means they have a caste system ( division of labor), with a specialized caste that lays eggs (queen) and a worker caste of sterile females. There are several reasons that they are considered pests. About 60% of people living in areas where fire ants occur are stung every year. Of these, about 1% have some degree of allergic reaction ( called anaphylaxis) to the sting. Their large mounds are unsightly and can damage mowing equipment. Fire ants sometimes enter electrical and mechanical equipment and can short out switches or chew through insulation. Finally. as fire ants move into new areas, they reduce diversity of native ants and prey on larger animals such as ground-nesting birds and turtles.Even though fire ants are pests in many circumstances, they can actually be beneficial in others. There is evidence that their predatory activities can reduce the numbers of some other important pests. In cotton, for example, they prey on important pests that eat cotton plants such as bollworms and budworms. In Louisiana sugarcane, an insect called the sugarcane borer used to be a very important pest before fire ants arrived and began preying on it. Fire ants also prey on ticks and fleas.Whether fire ants are considered pest or not depend on where they are found, but one thing is sure—we had best get used to living with them. Eradication attempts in the 1960s and 1970s failed for a number of reasons, and scientists generally agree that complete elimination of fire ants from the United States is not possible. A new, long-term approach to reducing fire ant populations Involves classical biological control. When fire ants were accidentally brought to the United States, most of their parasites and diseases were not. Classical biological control involves identifying parasites and diseases specific to fire ants in South America, testing them to be sure that they don ’ t attack or infect native plants or animals and establishing them in the Introduced fire ant population In the United States. Since fire ants are about 5 to 7 times more abundant here than in South America, scientists hope to reduce their numbers using this approach. The word "caste" ( line 4, paragraph 1 ) most probably means（）

A. throwing.
B. castle.
C. rank.
D. level.