<span>In the context of this passage (or any other passage, for that matter), the phrase a rule of thumb is best understood to mean a rule that usually works. The correct answer is D. It is a principle that is not necessarily always reliable, but works most of the time. Unfortunately, the phrase comes from domestic violence, where it was stated that a man could beat his wife as long as the stick he used for beating her wasn't thicker than his thumb. </span>
