词汇（每题1分，共5题，5分。）
用所给单词的适当形式填空。
Can you tell me the ___________(important) of knowledge(知识)?
It’s not easy to ______________(communication) with a stranger.
Tell you the ___________ (true), I’ve never seen him before.
I have two ___________ (old) sisters.
She often falls _________ (sleep) in class.