Simplicity, simplicity, simplicity! i say, let your affairs be as two or three, and not a hundred or a thousand; instead of a million count half a dozen, and keep your accounts on your thumb-nail. which best describes the purpose of the “thumb-nail” imagery? it illustrates thoreau’s belief that a person’s responsibilities should include some manual labor. it illustrates thoreau’s belief that most people waste their time attending to their bodies’ needs. it emphasizes thoreau’s belief that people should carefully choose their commitments. it emphasizes thoreau’s belief that people should carefully record their accounts.

answer: it emphasizes thoreau’s belief that people should carefully record their accounts.

in this excerpt, thoreau tells people how he thinks that they should handle their affairs. thoreau tells readers that people should keep their accounts on their "thumb-nail." the meaning of this phrase is that people should carefully record their accounts. by using the idea of the thumb-nail, thoreau emphasizes the idea that people should have their accounts organized and easily available.

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).