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
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The word "these" in line 13 refers to（）
A. seeds
B. river valleys
C. the Midwest and the Southeast
D. experimentation and domestication
According to the passage, which of the following was a possible motive for the cultivation of plants in eastern North America（）
A. Lack of enough wild food sources
B. Tile need to keep trees from growing close to settlements
C. Provision of work for an increasing population
Desire for the consistent availability of food
The word "intriguing" in line 5 is closest in meaning to（）
A. changing
B. perfect
C. visible
D. fascinating
The word "Ultimately" in line 14 is closest in meaning to（）
A. frequently
B. normally
C. whenever possible
D. in the end
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