OG6 阅读 Q26

Choice A is the best answer. The passage explains that as part of their research the scientists "made half the plants more fragrant by tucking dimethoxybenzene-treated swabs deep inside their flowers. Each treated flower emitted about 45 times more fragrance than a normal one" (lines 35-39,"throughout...half of"). In this context, a flower that was "treated" would be one that was changed or altered. 
Choices B, C, and D are incorrect because in the context of a flower having a compound like dimethoxybenzene added to it, the word "treated" means changed or altered, not returned to normal (choice B), given (choice C), or kept for future use (choice D).