【单选题】

The smooth operation of an ant colony depends on ten to twenty different signals,　　　 most of which are pheromones (chemical signals triggering behavioral responses). It is　　　 estimated that red fire ants employ at least twelve different chemical signals. The simples of these is the carbon dioxide from the respiration of an ant cluster, a chemical that acts(5)　 a pheromone to promote aggregation. Workers move toward a source of carbon dioxide,　　　 resulting in solitary ants moving to join a group.At the other extreme, the most complex　　　 of the fire ants’’ signals is probably colony odor, by which the workers of a particular　　　 colony or nest identify another worker as local or foreign.Each ant nest has its own odor　　　 as a result of its location, history, and local food supply. The resident ants pick up this(10)　 odor on their bodies, so that ants of the same species, but from different nests, have　　　 different colony odors. This allows ants to identify intruders and maintain colony　　　 integrity.　　　 Fire ants also make use of an alarm pheromone to alert workers to an emergency,　　　 and their scouts lay down a trail pheromone as a guide during mass migrations.

A、fire ant(15)　 queen emits a chemical signal that identifies her to the colony’’s workers. They respond　　　 by scurrying to gather around her. The decomposing corpse of a dead ant also generates　　　 a signal, to which workers respond by eliminating the corpse from the nest.　　　 Ants provide examples of both public (accessible to other species) and private　　　 messages. One of their most important private messages concerns food, for a food source(20)　 is worth keeping secret.Each species marks its trails with signals that are meaningless to　　　 others, so that an ant crossing a trail left by another ant species typically notices nothing.　　　 On the other hand, a secret signal to mark a dead body is unnecessary. Many kinds of ants　　　 perceive a natural decomposition product of dead insects as a signal to remove a corpse.If an outsider recognizes this message and moves the body, no harm is done.The word "cluster" in line 4 is closest in meaning to
A、organ
B．activity
C．group
D．cycle