根据句意及首字母提示完成单词。

1. There aren't a______ students in the room.

2. The tall b______ is a hotel and there are a lot of rooms in it.

3. There is a big g______ in our school and we often play basketball in it.

4.  —Where's my s______ book? 

     —It's in the lab.

5. Don't sit in f______ of me. I can't see the blackboard.

婚姻可以被撤销吗？