用所给词的适当形式填空。
1. Wood _________(come) from trees.
2. We should make _______(paper) bags and _________(glass) bottles.
3. Water is ________(use).We drink water and use water __________(clean) things every day. We should keep the water in the river __________(clean).
4. Sam starts _________(draw).
5. We shouldn’t __________(litter) in the park.
6. I know how ________ (protect) the Earth.
7. Let’s make a _________(post) and tell them about it.
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