牙周组织的主要防御机制有（）
A.上皮屏障
B.吞噬细胞
C.龈沟液
D.唾液
E.以上都是
参考答案：E
——I’m sorry to interrupt you. Please go on.
——Where was I?
——You ______ you didn’t like going shopping alone..
A．had been saying
B．had said
C．said
D．were saying
胶体物质有何特点？
