根据中文提示及上下文, 写出正确的单词。

The Earth is our home. There are rivers and （森林）, mountains and fields. The Earth gives us air, water and food. So we should （保护）it. What can we do? We can（种植）more trees because they help keep the air clean. Too much （塑料）is bad for the Earth. So we should use paper bags and glass bottles.