资料题：Which of the following is a sixth-generation aircraft（  ）。

While popular in the U.S., the April.Fool's Day tradition is even more prevalent in European countries, such 8s France and Great Britain.Although the roots of the traditional tricks are unclear, the French and the British.both have claims on the origin of the celebration.One theory holds that the first April Fool's Day was on April 1 of the year when King of France instituted the new calendar.This new system placed the day that had formerly been the first day of a new year on April 1.Many people were reluctant to adjust to the new calendar and continued to celebrate New Year' s Day on what had become the first day of April.Thus, they become the first April fools.Others began to give gag gifts on the day to mock the foolishness ofthose who continued to celebrate the new year on April 1.An English story about the day, however, holds that it began sometime during the 1200s.At the time, King John of England was in the habit of making a road out of nearly every path hewalked regularly.The citizens of one particular farm village were aware of this.To avoid havingtheir green meadows...and pastures(牧场) disturbed with one of the king's roads, they built afence that prevented the king from walking through their countryside.The king sent a group ofmessengers to inform the villagers that they must remove the barrier.Upon hearing that the kingwas planning to do this, however, the villagers developed a plan of their own.When the messengers arrived, they found what appeared to be a community of lunatics(精神病患者), with people behaving in a bizarre manner, throwing thing and running around wildly .The messengers,alarmed at what they had found, reported to King John that these people were so mad as to be beyond punishment.So, the villagers saved their farmland by tricking the King.In Great Britain, tradition only allows April Fool's tricks from midnight to noon on April 1.Those who try to play tricks in the afternoon become the fools themselves.