The stimulus states that some experts hired to testify in court may not necessarily be the most knowledgeable, but rather merely more persuasive than other experts. The stimulus's evidence for this is that experts hired to testify in court are evaluated by juries in terms of their ability to present the steps by which they arrived at their conclusion clearly and confidently, so those hiring the experts might value this skill of theirs over their actual level of expertise.
So ultimately we are explaining why a certain group of people (in the stimulus's case: experts hired to testify in court) may not necessarily the best at the quality they are known for (i.e. may not necessarily be the most knowledgable experts), because of a separate quality that merely helped them joined this group in first place (i.e. the experts are evaluated by juries in terms of the ability to present the steps by which they arrived at their conclusions clearly and confidently).
Answer choice (A) closely conforms to this principle. It tells us that successful politicians may not necessarily be the ones who best understand how to help their country (a quality commonly attributed to politicians), because they may instead merely be those who are highly skilled at conducting election campaigns and, as such, win their elections over those politicians who may better understand how to help their countries. As in the stimulus, answer choice (A) explains why a certain group of people may not necessarily be the best at a quality commonly attributed with that title because of a separate quality that merely helped them attain their title.
Answer choice (B), however, does not similarly conform to the principle in the stimulus. It merely explains that trial lawyers often employ techniques used by actors to influence the emotions of the jurors, and that some even expressly take drama for this purpose. No other quality is discussed.
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