medium · SAT Reading & Writing

When certain acacia trees are grazed by antelope, they flood their leaves with a bitter chemical within minutes, rendering the foliage unpalatable. Strikingly, untouched acacias standing downwind of the grazed trees begin producing the same chemical soon afterward, despite never having been browsed themselves. Researchers suspect that the grazed trees emit an airborne signal their neighbors detect. This evidence suggests that the untouched trees' production of the chemical is triggered not by direct grazing but by ___

1. the physical removal of leaves from the untouched trees by antelope.
2. the airborne compound released into the wind by nearby grazed trees.
3. a gradual, season-long shift in the chemistry of all the trees.
4. the complete disappearance of antelope from the surrounding area.

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