question: Which students participate in both Canoeing and Kayaking as their activities? Tell me their student ids.
#1 return students 
#2 return activities of  #1 
#3 return #1 where #2 is  Canoeing 
#4 return #1 where #2 is  Kayaking 
#5 return #1 of  both #3 and  #4 
#6 return student ids of  #5