根据句意及首字母提示完成句子。

1. Tina is such a clever student that she always comes at the t________ of her class.

2. Mother added some s________ to the soup, but it wasn't still salty enough.

3. You should c________ your answers .before you hand in your papers.

4. Ducks can swim while c________ can't.

5. You need different kinds of  i________ to make a delicious Beijing Duck.