Read the following passage, and mark the letter A, B, C, or D on your answer sheet to indicate the correct word for each of the  blanks from 19 to 28.

        Any change in one part of an ecosystem can cause changes  in other parts. Droughts, storms and fires can  change  ecosystems.  Some  changes  ___19___  ecosystems.  If  there  is  too  ___20___  rainfall, plants will not have enough water to live. If a kind of plant   dies off, the animals that ___21___ it may  also  die  or  move  away.   Some  changes  are  good  for  ecosystems.  Some  pine  forests  need ___22___ for the pine trees to reproduce. The seeds are sealed inside pinecones. Heat from a forest fire  melts  the  seal  and  lets  the  seeds___23___.  Polluting  the  air,  soil,  and  water  can  harm ecosystems.  Building  ___24___  on  rivers  for   electric  power  and  irr igation  can  harm  ecosystems ___25___  the   rivers.  Bulldozing  wetlands  and  cutting  down  ___26___  destroy  ecosystems. Ecologists  are  working  with  companies  and  governments  to  find  better  ways  of  ___27___  fish, cutting  down  trees,  and  building  dams.  T hey  are  looking  for  ways  to  get  food,  lumber,  and  other products for people ___28___ causing harm to ecosystems.

Điền vào số 27