Paragraph Completion - 2

A shrewd peasant was always well enough protected against impostors in the market place, and we have all sorts of businessmen who have made themselves excellent judges of phoniness without the benefit of a high-school diploma; but this kind of shrewdness goes along with a great deal of credulity. Outside the limited field within which experience has taught the peasant or the illiterate businessman his lessons, he is often hopelessly gullible.

Which of the options provides critical completion of the passage?