a rose is a kind of plant & 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 weeds use up the nutrients, then the rose will not have nutrients to grow  if weeds use up the nutrients, then the rose will not have nutrients to grow & the student wants to grow roses -> the student has to kill the weeds so that the rose gets nutrients to grow