根据句子的意思及首字母提示，用单词的适当形式填空。

1. The French book must b                   to Lily. She's the only one who's studying French.

2. Most children can't c                 on one thing for a long time.

3. You should exercise more so as to keep e                 .

4. There's a s                 smell in the room. Do you know what it is?

5. The girl ate too many c_________. So her teeth are bad .