用所给词的适当形式填空
1. We're doctors, so it's our _______ (duty) to save patients.
2. The road is _______ (crowd). Mr. Wang drives his car slowly.
3. What causes the _______ (ill)?
4. Did they enjoy _______ (they) in the party yesterday?
5. —Where is your mother, Beibei?
—She is busy _______ (answer) the telephone.