Rule: Vocabulary of Relationships. The fact that they 'have not spoken to each other since' implies the argument was very bitter and angry. 'Acrimonious' means (typically of speech or a debate) angry and bitter. This word accurately describes a friendship-ending argument. Options (a) 'amicable', (c) 'harmonious', and (d) 'cordial' are all words for friendly and peaceful interactions.

Rule: Vocabulary of Ecosystems. Rainforests are characterized by a dense, upper layer of trees. 'Canopy' is the term for the upper layer or habitat zone, formed by mature tree crowns. This is the correct term for the part of the rainforest being described. Options (b) 'desert', (c) 'tundra', and (d) 'savanna' are different types of biomes, not parts of a rainforest.

Rule: Business Vocabulary. Having a 'significant advantage over its competitors' because of a technology implies exclusive control or possession of it. 'Monopoly' means the exclusive possession or control of the supply of or trade in a commodity or service. This is the correct business term. Options (b) 'prohibition', (c) 'embargo', and (d) 'restriction' are all terms for limiting or banning something, which don't fit the context of gaining an advantage through possession.

Rule: Vocabulary of Communication. Explaining complex theories 'in simple, understandable terms' is the act of clear explanation. 'Exposition' is a comprehensive description and explanation of an idea or theory. This word perfectly describes the professor's skill. Options (a) 'obfuscation' (making something obscure), (c) 'equivocation' (using ambiguous language), and (d) 'complication' (making something more difficult) are all antonyms.

Rule: Verbs of Helping/Harming. A charity organization's purpose is to reduce or lessen suffering. 'Mitigate' means to make less severe, serious, or painful. This is the correct verb for the charity's work. Options (a) 'intensify', (b) 'prolong', and (d) 'increase' are all actions that would make suffering worse, which is the opposite of a charity's mission.

Rule: Vocabulary for Central Issues. The sentence is identifying the most essential or core part of the problem. 'Crux' means the decisive or most important point at issue. It correctly points to the central cause of the problem. Option (a) 'periphery' means the outer edge. Options (b) 'consequence' and (d) 'outcome' refer to the results of the problem, not its core.

Rule: Legal Vocabulary and Redundancy. A charge of 'perjury' means the witness lied under oath. The reflexive verb 'to perjure oneself' means to willfully tell an untruth when giving evidence to a court. So, she 'perjured herself'. While this makes the second half of the sentence slightly redundant, it is the only grammatically and logically correct option. Options (a) 'validated', (b) 'vindicated', and (c) 'corroborated' are all positive terms about proving something to be true or correct, the opposite of perjury.

Rule: Metaphorical Vocabulary. A 'vibrant and diverse community' is the result of cultures coming together and mixing. 'Confluence' literally means the junction of two rivers, but it is used metaphorically to mean an act or process of merging. It is an excellent word for the coming together of cultures. Options (a) 'segregation', (b) 'isolation', and (d) 'separation' are all antonyms, describing cultures being kept apart.

Rule: Vocabulary of Control. The purpose of a treaty concerning an 'arms race' would be to limit or control it. 'Curb' means to restrain or keep in check. This verb correctly states the goal of the treaty. Options (a) 'exacerbate', (b) 'promote', and (d) 'encourage' all mean to make the arms race worse or more intense.

Rule: Vocabulary of Humor. Humor based on 'subtle wordplay and irony' is dry and intelligent. 'Wry' humor is a type of humor that is dry, mocking, and understated. This is the perfect description. Options (a) 'broad', (b) 'slapstick', and (d) 'obvious' all describe types of humor that are very direct and physical, the opposite of subtle irony.