31.(A) Another crucial factor is a society’s immigration diversity. (B) Various factors have thus contributed to the phenomenon of smiling. (C) For most of recorded human history, the open smile has been deeply unfashionable. (D) A genuine smile indicates contentment, while a forced smile suggests underlying distress. (E) In fact, there is a well-known Russian proverb that goes, “Smiling with no reason is a sign of stupidity.”