根据句子意思，用括号内所给单词的适当形式填空。
The plane hasn’t arrived yet. Do you know the time of its____  (arrive)?
Mr. Sun felt cold and ____ ( frighten).
____ (luck), I got full marks in the last exam.
You needn’t take a notebook with you if you find it ____  (necessary).
Many people don’t know the____ (important) of the wetlands.