【单选题】

Many ants forage across the countryside in large numbers and undertake massmigrations; these activities proceed because one ant lays a trail on the ground for the othersto follow.As a worker ant returns home after finding a source of food, it marks the route by intermittently touching its stinger to the ground and depositing a tiny amount of trail(5)pheromone—a mixture of chemicals that delivers diverse messages as the context changes. These trails incorporate no directional information and may be followed by other ants in either direction.Unlike some other messages, such as the one arising from a dead ant, a food trail has to be kept secret from members of other species. It is not surprising then that ant species use(10)a wide variety of compounds as trail pheromones.Ants can be extremely sensitive to thesesignals. Investigators working with the trail pheromone of the leafcutter antAtta texana calculatedthat one milligram of this substance would suffice to lead a column of ants three times around Earth.The vapor of the evaporating pheromone over the trail guides an ant along the way, (15)and the ant detects this signal with receptors in its antennae.

A、trail pheromone will evaporate to furnish the highest concentration of vapor right over the trail, in what is called avapor space. In following the trail, the ant moves to the right and left, oscillating from sideto side across the line of the trail itself, bringing first one and then the other antenna intothe vapor space.As the ant moves to the right, its left antenna arrives in the vapor space. (20)The signal it receives causes it to swing to the left, and the ant then pursues this new courseuntil its right antenna reaches the vapor space. It then swings back to the right, and so weaves back and forth down the trail. The author mentions the trail pheromone of the leafcutter ant in line 11 to point out
A、how little pheromone is needed to mark a trail
B．the different types of pheromones ants can produce
C．a type of ant that is common in many parts of the world
D．that certain ants can produce up to one milligram of pheromone