How many years after rats were observed building nests in trees did an outbreak of plague occur that was attributed to the release of infected rats from ships arriving from India?
return rats were observed building nests in  trees ;return an outbreak of plague occur that was  attributed to the  release of  infected rats from  ships arriving from India ;return when  was  #1 ;return when  was  #2 ;return difference of #4 and  #3 in  years
['select', 'select', 'project', 'project', 'arithmetic']