牙骨质龋细菌入侵的主要通道是（）
A.牙骨质层板
B.穿通纤维
C.生长线
D.成牙骨质细胞突起
E.牙骨质细胞陷窝
参考答案：B
11. ______ she is young, she knows quite a lot.
A．When
B．However
C．Although
D．Unless
