根据括号内的汉语提示，完成句子。
The grandchild nods his head to show his ________(同意) to his grandpa.
I think __________(回收) can not only protect the environment but also save money.
Humans have come to realize the ___________ (重要)of protecting animals.
A large population can cause lots of __________ (困难)for the whole nation.
There are several chemical factories _______ (倾倒)waste water into the streams.
Some parents in less_________(发展) areas prefer boy or girls, don’t they?