What is killing the trees? Nobody knows exactly, but it is probably "air pollution" or "acid rain". Factories, power station and cars made tons of smoke into the air. This smoke contains sulfur dioxide and nitrogen oxide. These substances mix with water vapor in the atmosphere and from sulfuric acid.
Sunlight turns this acids into poisonous oxidants which falls in rain or snow onto the trees. What can be done about acid rain? Why do not governments do something to save the trees? Unluckily, acid rain is not rain and does not always fall on the countries, which produce the pollution. The wind carries the poisonous substances from one country to another. Why should the people of one country pay to save another country's tree?
The word "poisonous" in this passage can be replaced by _____________.
pleasant
powerful
harmful
healthy