一．单项选择 （每空1分，共20分）
1. ___ they suddenly disappeared still remains a mystery.
A. That B. Where C. Why D. What
2. Someone is ringing the doorbell. Go and see ____.
A. who is he B. who he is C. who it is D. who is it
3. Parents are taught to understand ___ important education is to their children’s future.
A. how B. that C. so D. such
4. ---Are you still thinking about yesterday’s game?
---Oh, that’s ____.
A. whatever I feel excited about B. what makes me feel excited
C. when I feel excited D. how I feel about it
5. I want to know ___ the thief was caught on the spot.
A. whether B. that C. which
