听觉信号，长声为三秒，短声为一秒，音响间隔为一秒，重复鸣示时，须间隔（）以上。
A、3s
B、4s
C、5s
D、6s
参考答案：C
Our family were thinking about __________ Hainan Island at this time yesterday.
[ ]
A. to visit
B. Visited
C. visiting
D. visit
无牙颌患者的口内特征为（）
A.牙列缺失
B.牙列缺损
C.牙冠缺损
D.牙体缺失
E.牙体缺损
