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.

获得性免疫缺陷综合征

A．HIV感染
B．C₁INH缺乏
C．血清IgA极低，IgG、IgM水平正常
D．Bruton酪氨酸激酶基因缺乏
E．CD₁₈基因突变