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Questions 20-29

Many ants forage across the countryside in large numbers and undertake mass migrations; these activities proceed because one ant lays a trail on the ground for the others to 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 these signals. Investigators working with the trail pheromone of the leafcutter ant Atta texana calculated that 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 a vapor space. In following the trail, the ant moves to the right and left, oscillating from side to side across the line of the trail itself, bringing first one and then the other antenna into the 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 course until its right antenna reaches the vapor space. It then swings back to the right, and so weaves back and forth down the trail.

20. What does the passage mainly discuss?

(A) The mass migration of ants

(B) How ants mark and follow a chemical trail

(C) Different species of ants around the world

(D) The information contained in pheromones

21. The word “forage” in line 1 is closest in meaning to

(A) look up

(B) walk toward

(C) revolve around

(D) search for food

22. The word “intermittently” in live 4 is closest in meaning to

(A) periodically

(B) incorrectly

(C) rapidly

(D) roughly

23. The phrase “the one” in line 8 refers to a single

(A) message

(B) dead ant

(C) food trail

(D) species

24. According to the passage, why do ants use different compounds as trail pheromones?

(A) To reduce their sensitivity to some chemicals

(B) To attract different types of ants

(C) To protect their trail from other species

(D) To indicate how far away the food is

25. 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

26. According to the passage, how are ants guided by trail pheromones?

(A) They concentrate on the smell of food.

(B) They follow an ant who is familiar with the trail

(C) They avoid the vapor spaces by moving in a straight line.

(D) They sense the vapor through their antennae.

27. The word “furnish” in line 16 is closest in meaning to

(A) include

(B) provide

(C) cover

(D) select

28. The word “ oscillating “ in line 17 is closest in meaning to

(A) falling

(B) depositing

(C) swinging

(D) starting

29. According to the passage, the highest amount of pheromone vapor is found

(A) in the receptors of the ants

(B) just above the trail

(C) in the source of food

(D) under the soil along the trail

Key: B DAACA DBCB