The word choice for the first blank is defined in the phrase “the origin and development” of word meanings. The definition that determines the blank applies only to (B) etymology, the correct answer. The word (A) toxicology refers to the study of poisons and is therefore inappropriate. A (C) taxonomy is a system of scientific classifications. The sense of the first sentence is that a dictionary showing the development of word meanings “offers proof” of a certain type of language, one in which words “change.” A language that is (E) living is one that changes and develops, which makes that the correct answer here. A (D) nascent language is one that has just been born, so rule that out. A language that is (F) faltering is “hesitant” or “unsteady,” which makes no sense in the context. The second sentence includes the construction “not only.  .  . but,” which serves as a clue that you need a word that is more extreme than “change.” The word (G) reverse is correct here because it carries the right weight and matches the example. The word (H) amend is synonymous with the word “change” and is therefore inappropriate. Choice (I) exchange makes some sense, since words might exchange meanings. However, this answer doesn’t carry the idea of an extreme change, nor does it fit the example of “nice,” used to illustrate it.