阅读理解。
Plant
ing trees around poultry (家禽) farms can improve air and water quality - and relations with
neighbors. Research has shown that just three rows of trees near poultry houses can reduce the release
of dust and ammonia (氨). They can also reduce the strong smell of ammonia gas.
The trees capture dust, ammonia and smells in their leaves. They can also reduce energy use. They
also provide shade from the sun, so they reduce cooling costs in summer. And they act as a windbreak,
so they reduce heating costs in winter. Trees can also improve water quality around farms by removing
pollutions from soil and groundwater.
Several years ago, people were objecting to the smell of poultry farms on the Delmarva Peninsula in
the eastern United States. Delmarva is where the states of Delaware, Maryland and Virginia come
together. Two thousand farms there can each house an average of seventy-five thousand chickens.
Traditionally the farms used windows to provide fresh air in the chicken houses. Farmers rarely
planted trees or tall crops around the buildings, so there would be no barrier to the airflow.
But then in the 60s, farms began to use mechanical ventilation (通风) systems. Instead of windows,
the mechanical systems used tunnel fans to circulate (使…循环) air. The fans directed airflow from the
poultry houses toward the homes of neighbors.
Researchers began dealing with the problem in 2000. They found that over a period of six years,
planting three rows of trees reduced total dust and ammonia by more than half. And they found that
smells were reduced by 18%.
Farmers may think trees will take too long to grow and be effective. But some trees can grow quickly. At least one-third of the Delmarva farms have planted trees, technically known as vegetative
environmental buffers. The idea offers a way to cut pollution, save money and energy, and make the
neighbors happy.
1. 
How did the mechanical ventilation system work?
_____________________________________________________
2. The number of chickens raised on the poultry farms in Delmarva is up to ____________________.
3. The benefits of planting trees around poultry farms are _________________.
4. What is the main topic of the passage?
_____________________________________________________
