用 must, oughtn't to, don't have to, mustn't, needn't或  have to填空。

1. You _____ go on working because you look so tired now.

2. The weather turned out to be fine yesterday. I ______ have had the trouble to carry my umbrella

    with me.

3. If you earn more than ~5,000, you _____ pay tax.

4. You ______ play now !  You shall finish your homework first!

5. You can't give up. You ______ try to get over the difficulties.

肿瘤细胞间细胞连接的特征不包括（）

A.缝隙连接减少

B.缝隙连接增加

C.桥粒明显减少

D.细胞内生性桥粒的形成

E.半桥粒的松解变性