皮肤的表皮细胞经常脱落而成为皮屑,但皮肤并不会因此而越来越薄,这是因为_______细胞有很强的分裂增生能力。
A.角质层
B.生发层
C.真皮层
D.皮下脂肪细胞
答案:B
He ____ writing the paper now. He hadn't written a single word when I left him ten minutes ago. [ ]
A. shouldn't be
B. can't have finished
C. can't be
D. mustn't have finished
B-ISDN的协议参考模型每个面又分为3层,即( )。
A.物理层、ATM层和删适配层
B.物理层、ATM层和拆装层
C.物理层、ATM尾和会聚层
D.物理层、ATM层和公共会聚层