what is africa's fourth largest river?
return africa ;return rivers of #1 ;return the  largest of #2 ;return #2 besides  #3 ;return the  largest of #4 ;return #2 besides  #5 ;return the  largest of #6 ;return #2 besides  #7 ;return the  largest of #8
['select', 'project', 'aggregate', 'discard', 'aggregate', 'discard', 'aggregate', 'discard', 'aggregate']