根据句意，用括号内所给单词的适当形式填空。
You ________ (simple) can’t imagine how terrible the earthquake is!
Flying across the Atlantic for the first time was a great ________ (achieve).
A true friend can help you get out of ________ (sad) when you’re unhappy.
You should think________ (two) about this, or you’ll regret (后悔) in the future.