47．According to the passage，when did Herakleion and Eastern Canopus disappear?
48．As Stanley said,if you want to know how，when or why Herakleion and Eastern Canopus disappeared，there was／were____________to refer to．
49．Stanley thought that_________was to blame for the citiescollapse in the middle of the eighth century A.D．
50．What found below the waters could show that the cities were once the destinations for religious pilgrims?
51．According to the author，the fortunes of Herakleion and Eastern Canopus declined in__________________.