青少年牙周炎在疾病早期常表现为（）。
A．牙龈炎症重
B．牙槽骨致密
C．牙齿松动和移位
D．口腔卫生差，牙石多
E．好发牙位为第二磨牙
参考答案：C
We all feel it is Jack as well as his wife that ____ for their son's bad behaviour at school. [ ]
A. are to blame
B. is to be blamed
C. are to be blamed
D. is to blame
可分为RAM和ROM两种类型的存储器是______。
A．外存储器 B．内存储器C．高速缓存 D．闪存
