Explanation

The sentence describes how river diversion "tends to ______ a host of unintended and often deleterious consequences." The phrase "repercussions that planners rarely anticipate" reinforces that these are unexpected, unplanned outcomes. The blank must mean cause, produce, generate, or bring about.

The correct pair must be tight synonyms that both describe causing or producing something, often with a connotation of unintended or sudden emergence.
- A. precipitate - To cause to happen suddenly, unexpectedly, or prematurely. "Tends to precipitate a host of unintended consequences" perfectly captures the sudden, unplanned emergence of negative effects. Correct.
- B. circumvent - To bypass, avoid, or find a way around. The diversion does not avoid consequences - it causes them. Incorrect.
- C. nullify - To cancel, make void, or negate. The diversion does not cancel consequences - it produces them. Incorrect.
- D. exacerbate - To worsen, intensify, or aggravate. The diversion does not worsen pre-existing consequences - it creates new ones. Incorrect.
- E. beget - To cause, produce, or give rise to; to bring into existence. "Tends to beget a host of unintended consequences" produces a sentence virtually identical in meaning to the one with "precipitate." Correct.
- F. ameliorate - To improve, make better, or alleviate. The consequences are "deleterious" - the diversion causes harm, not improvement. Incorrect.