Task: Question Generation
Input (English): #1 return flights 
#2 return #1 from boston 
#3 return #2 to  san francisco 
#4 return #1 from san francisco 
#5 return #4 to  boston 
#6 return the  aircraft of #3 
#7 return the  aircraft of #5 
#8 return #6 ,  #7
Output (English): list types of aircraft that connect boston and san francisco