In the context of this passage, the phrase a rule of thumb is best understood to mean a
a. rule to be avoided.
b. rule for an emergency.
c. smart way to use your thumb.
d. rule that usually works

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.