【推荐1】Many volunteers prefer to take part in something ________ (challenge). （所给词的适当形式填空）

【推荐2】We can surely overcome these ________ (challenge) only if we are closely united. (所给词的适当形式填空)

【推荐3】Destruction of the environment is one of the most serious_____ (challenge) we face. （所给单词适当形式填空）

【推荐1】He talked with his parents without ________（refer）to the fact that he had registered for an institute.（所给词的适当形式填空）

【推荐2】用settle的适当形式填空
1. —Why do you look sad?
—There are so many problems remaining ___________.（settle）
2. With a lot of difficult problems ___________（settle）, the newly-elected headmaster is having a hard time.
3. Early ___________ （settle）in this area lived a difficult life.
4. We hope for a last ___________ （settle）of all these troubles.
5. They ___________ （settle）in Shanghai last year.
6. It’s said that Samuel ___________   the first permanent __________ (settle) in present Canada.
7. It was a rainy morning and the children, mainly boys with various learning difficulties, refused _______ (settle) for the start of the lesson.

【推荐3】While attempting to talk to the __________ (operate), Mrs Taylor discovered that she could not speak, not hearing an answer. (所给词的适当形式填空)