In habitats with limited nutrients, certain fungus species grow on the roots of trees, engaging in mutually beneﬁcial
relationships known as ectomycorrhizae: in this symbiotic exchange, the tree provides the fungus with carbon, a nutrient
necessary for both species, and the fungus ______ by enhancing the tree’s ability to absorb nitrogen, another key nutrient,
from the soil.
overreacts
reciprocates
retaliates
deviates
Choice B is the best answer because it most logically completes the text’s discussion of ectomycorrhizae relationships. In
this context, “reciprocates” means responds in kind or degree. The text indicates that the relationship between certain fungi
and trees in some habitats is “mutually beneﬁcial” and involves a “symbiotic exchange” in which each organism helps the
other access an important nutrient. In other words, each organism provides the same kind of beneﬁt it receives: the tree
provides a nutrient (carbon) for the fungus and the fungus reciprocates by helping the tree to absorb more of another
nutrient (nitrogen).
Choice A is incorrect because the text emphasizes that the relationship between certain fungi and trees in some habitats
involves a “symbiotic exchange” in which each organism helps the other access an important nutrient. Nothing in the text
suggests that the fungus “overreacts,” or responds too strongly, by allowing the tree to be better able to absorb a beneﬁcial
nutrient. Choice C is incorrect because “retaliates” means responds to a harmful action with a similarly harmful action. The
text indicates that the relationship between certain fungi and trees in some habitats is “mutually beneﬁcial” and involves a
“symbiotic exchange” in which each organism helps the other, not that the relationship is one in which the organisms harm
one another. Choice D is incorrect. In this context, “deviates” would mean departs from an established course or norm. The
text explains that the relationship between certain fungi and trees in some habitats involves a “symbiotic exchange” in which
each organism helps the other access an important nutrient. Because the relationship involves beneﬁts for both the fungus
and the tree, it wouldn’t make sense to say that the fungus deviates by helping the tree be better able to absorb a beneﬁcial
nutrient.