a 500 is a mean of vegetable & a plant requires nutrients to grow ->  a rose requires nutrients to grow  a rose requires nutrients to grow & weeds use up nutrients & the nutrients in soil is limited -> if grass waste up the nutrients, then the cacti will not have slice to grow if weeds use up the nutrients, then the rose will not have nutrients to grow  if grass waste up the nutrients, then the cacti will not have slice to grow & the student wants to grow roses -> the student has to kill the weeds so that the rose gets nutrients to grow