Read the following article and select the best answer. Article: Planting trees around poultry 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 . They can also reduce the strong smell of ammonia gas. The trees take dust, ammonia and odors in their leaves. 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 odor of poultry farms on the Delmarva Peninsula in the eastern United State s. 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 farms began to use new ventilation systems. Instead of windows, the new systems used tunnel fans to circulate air. The fans directed airflow from the poultry houses toward the homes of neighbors. Researchers began dealing with the problem in two thousand. 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 eighteen percent. 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. Question: The farms on the Delmarva Peninsula used to use _ in chicken houses to provide fresh air. - trees - tunnel fans - windows - poles
The answer to this question is:
windows