Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 19 to 23.

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 little rainfall, plants will not have enough water to live. If a kind of plant dies off, the animals (20)......feed on it may also

die or move away. Some changes are good for ecosystems. Some pine forests need fires for the pine trees to reproduce. The seeds are sealed inside pinecones. Heat from a forest fire melts the seal and lets  the  seeds  (21)........  Polluting  the  air,  soil,  and  water  can  harm ecosystems.  Building  (22).......on rivers  for  electric  power  and  irrigation  can  harm  ecosystems  around  the  rivers.  Bulldozing wetlands and cutting down forests destroy ecosystems. Ecologists are working with companies and governments to find better ways of (23) ...... fish, cutting down trees, and building reservoirs. They are looking for ways to get food, lumber, and other products for people without causing harm to ecosystems.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blank 19.